drm/amd/display: fix gamma setting

Adding gamma changed check as condition for affected plane.
We ignored adding plane as affected if modeset was not required.
But for color management change we still need it.

Signed-off-by: Roman Li <Roman.Li@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>