drm/amd/display: To update backlight restore mechanism

[Why]
Cached backlight is never being updated since panel_cntl specific
registers were moved from abm to panel_cntl.

[How]
Update cached backlight in set_abm_immediate_disable as what we used to
do. Also, update the priority of backlight restore mechanism so that
cached backlight has the highest priority since it is always correct.

Signed-off-by: Camille Cho <Camille.Cho@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>