drm/amd/display: Fix dereference-before-check for dc_link

The function dereferences amdgpu_dm_connector->dc_link early to
initialize verified_link_cap and dc, but later still checks
amdgpu_dm_connector->dc_link for NULL in the analog path.

This late NULL check is redundant, introduce a local dc_link pointer,
use it consistently, and drop the superfluous NULL check while using
dc_link->link_id.id instead.

The function uses dc_link at the very beginning without checking if it
is NULL. But later in the code, it suddenly checks if dc_link is NULL.

This check is too late to be useful, because the code has already used
dc_link earlier. So this NULL check does nothing.

We simplify the code by storing amdgpu_dm_connector->dc_link in a local
dc_link variable and using it throughout the function. Since dc_link is
already dereferenced early, the later NULL check is unnecessary and is
removed.

Fixes the below:
amdgpu_dm_connector_get_modes():
variable dereferenced before check 'amdgpu_dm_connector->dc_link'

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c
8845 &amdgpu_dm_connector->dc_link->verified_link_cap;
8846 const struct dc *dc = amdgpu_dm_connector->dc_link->dc;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Dereference
...

8856
8857 if (amdgpu_dm_connector->dc_sink &&
8858 amdgpu_dm_connector->dc_link &&
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Checked too late.
Presumably this NULL check could be removed?
...

Fixes: d46e422f65ae ("drm/amd/display: Cleanup uses of the analog flag")
Reported by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Timur Kristóf <timur.kristof@gmail.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Ivan Lipski <ivan.lipski@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>