drm/amd/display: use HW hdr mult for brightness boost

In MPO scenario when playing SDR clip in HDR desktop mode, Win is
boosting desktop and requests driver to boost MPO. But driver boosting
is currently done in regamma which is stream property and thus shared
between grph and video.

Redesigning the boosting in RV: use CM_HDR_MULT register which was added
for this scenario. It also has the benefit that it can be done in HIRQL.

Signed-off-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>