drm/amd/display: Grab dc_lock before detecting link

[Why & How]
There is chance we change dc state while calling dc_link_detect().
As the result of that, grab the dm.dc_lock before detecting link.

Reviewed-by: Hersen Wu <hersen.wu@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>