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

[ Upstream commit 8ce504b9389be846bcdf512ed5be8f661b3bf097 ]

[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>
Signed-off-by: Sasha Levin <sashal@kernel.org>