drm/amd/display: fix brightness level after resume from suspend

Adding missing call to cache current backlight values.
Otherwise the brightness resets to default value on resume.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>