drm/amd/display: PQ regamma end point

[WHY & HOW]
PQ has a fixed range of 0-10,000 nits.
Using 1=80 nits normalization, PQ should map to 1.0 for X=125.0
HW LUT used for interpolation does not have X=125 so it's interpolated.
However, we cap Y to 1 for all X>=125.

The closest larger HW point is 128.
What we want is Y(128) such that interpolation through 125 gives 1.0.
Such value is ~1.00256.
Another change is to hardcode PQ table, we pretty much always have 1=80
normalization so the table can be static.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Krunoslav Kovac <krunoslav.kovac@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>