drm/amd/display: Allow inverted gamma

[why]
There's a use case for inverted gamma
and it's been confirmed that negative slopes are ok.

[how]
Remove code for blocking non-monotonically increasing gamma

Signed-off-by: Aidan Yang <Aidan.Yang@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Reza Amini <Reza.Amini@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>