drm/mediatek: Move DP training to hotplug thread

By adjusting the order of link training and relocating it to HPD,
link training can identify the usability of each lane in the current link.

It also supports handling signal instability and weakness due to
environmental issues, enabling the acquisition of a stable bandwidth
for the current link. Subsequently, DP work can proceed based on
the actual maximum bandwidth.

It should training in the hpd event thread.
Check the mode with lane count and link rate of training.

If we're eDP and capabilities were already parsed we can skip
reading again because eDP panels aren't hotpluggable hence the
caps and training information won't ever change in a boot life

Therefore, bridge typec judgment is required for edp training in
atomic_enable function.

`mtk_dp_parse_capabilities` is related to DP training, it is
used in `mtk_dp_hpd_event_thread` before DP training, and then only
used by eDP when read edid.

-Modify part of in `mtk_dp_bridge_atomic_disable`
if (mtk_dp->train_info.cable_plugged_in) {
drm_dp_dpcd_writeb(&mtk_dp->aux, DP_SET_POWER, DP_SET_POWER_D3);
usleep_range(2000, 3000);
}

/* power off aux */
mtk_dp_update_bits(mtk_dp, MTK_DP_TOP_PWR_STATE,
DP_PWR_STATE_BANDGAP_TPLL,
DP_PWR_STATE_MASK);

-Modify part of in `mtk_dp_aux_panel_poweron(mtk_dp, false);`
if (pwron) {
....
} else {
/* power off panel */
drm_dp_dpcd_writeb(&mtk_dp->aux, DP_SET_POWER, DP_SET_POWER_D3);
usleep_range(2000, 3000);

/* power off aux */
mtk_dp_update_bits(mtk_dp, MTK_DP_TOP_PWR_STATE,
DP_PWR_STATE_BANDGAP_TPLL,
DP_PWR_STATE_MASK);
}

The `mtk_dp_aux_panel_poweron` function fails to align.
Within the `mtk_dp_hpd_event_thread`, if DP is disconnected,
the `mtk_dp_aux_panel_poweron` function will write from `aux`
to `DPRX`, causing a failure and thus preventing symmetry.

This shows the current timings after the DP cable is plugged in,
as well as the modified timings.

current timings: Fix timings:

mtk_dp_hpd_event_thread() mtk_dp_hpd_event_thread()
(including DP link training)
| |
... ...
mtk_dp_bridge_mode_valid() mtk_dp_bridge_mode_valid()
|
... ...
mtk_dp_bridge_atomic_check() mtk_dp_bridge_atomic_check()
|
... ...
mtk_dp_bridge_atomic_enable() mtk_dp_bridge_atomic_enable()
(including DP link training)

PS:
1. "..." represents ommited steps;

2. `mtk_dp_bridge_mode_valid()` calculates the bandwidth using the
current lane count and link rate, and then filters each mode to
determine if it supports returning a status.

3. In the `drm_display_mode_to_videomode(&crtc_state->adjusted_mode,
&mtk_dp->info.vm);` function, within the `mtk_dp_bridge_atomic_check()`
function, `adjusted_mode` sets the currently selected display mode for
the DRM.

4. DP link training tests the signal conditions of the link between
DPTX and DPRX, and selects the lane count and link rate that meet
the signal conditions.

5. For example, the platform support DP 4lane 5.4G,
but panel A support DP 2lane 5.4G.

This is a time sequence:

a).Plug in panel A. According to the platform, it can output 4K60Hz.
b).Timing mode set 4K 60Hz(Including in mtk_dp_bridge_atomic_check
function).
c).Atomic enable(Based on panel A ability, training pass 2lane
5.4G).
d).Finally, due to 2lane 5.4G bandwidth limitation, the platform
cannot output 4K 60Hz, resulting in a black sreen.

If apply this patch.

a).Plug in panel A.
b).Training pass 2lane 5.4G
c).Timing mode set 2K 60Hz(Based on the 2lane 5.4G bandwidth limit
and including in mtk_dp_bridge_atomic_che

Signed-off-by: Liankun Yang <liankun.yang@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20251223061755.7717-1-liankun.yang@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>