drm/amd/display: programming last delta in output transfer function LUT to a correct value

[Why]
Currently DAL programs negative slope for the last point of output
transfer function curve.

[How]
Applying a check for the last PWL point for RGB values not to be
smaller than previous. If smaller, initialize the last point values
to a sum of previous PWL value and previous PWL delta;

Signed-off-by: Vladimir Stempen <vladimir.stempen@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>