drm/amd/display: program default output gamma

program default output gamma if no user specific gamma
parameters passed.

Signed-off-by: hersen wu <hersenxs.wu@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>