drm/amd/display: Don't disable DPCD mst_en if sink connected

[WHY]
User may connect mst dock with multi monitors and do quick unplug
and plug in one of the monitor. This operatioin may create CSN from
dock to display driver. Then display driver would disable and then enable
mst link and also disable/enable DPCD mst_en bit in dock RX. However,
when mst_en bit being disabled, if dock has another CSN message to
transmit then the message would be removed because of the disabling of
mst_en. In this case, the message is missing and it ends up no display in
the replugged monitor.

[HOW]
Don't disable mst_en bit when link still has sink connected.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Peichen Huang <PeiChen.Huang@amd.com>
Signed-off-by: Chenyu Chen <chen-yu.chen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>