drm/amd/display: Add driver-side parsing for CM

Although 4 unique register values exist for gamma modes, two are
actually the same (the two RAMs) It’s not possible for caller to
understand this HW specific behavior, so some parsing is necessary
in driver

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>