drm/amd/include: Add OCSC registers

Add registers for handling Post Gamma Color Blending (OCSC), which is
useful for conversion from RGB->YUV for HDMI.

Reviewed-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>