drm/amd/display: Add set backlight to hw sequencer.

[Why & How]
Add set backlight to hw sequencer, dmu communication will
be handled in hw sequencer for new asics.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>