drm/amd/display: Use DGAM ROM or RAM

[Why]
Optimize gamma programming

[How]
Use ROM for optimization when it is possible.
Use RAM only when it is necessary.

Signed-off-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>