drm/amd/display: fix gamma not being applied correctly

[why]
Gamma was always being set as identity on SDR monitor,
leading to no changes in gamma. This caused nightlight to
not apply correctly.

[how]
Added a default gamma structure to compare against
in the sdr case.

Signed-off-by: Murton Liu <murton.liu@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>