drm/amd/display: Adding support for VESA SCR

[HOW&WHY]
Write DPCD 721 bit 7 to high, and
the appropriate luminance level
to DPCD 734-736 if bit 4 from DPCD register
734 is high, indicating that the panel
luminance control is enabled from the panel side.

Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Iswara Nagulendran <Iswara.Nagulendran@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>