drm/amd/display: Fix max brightness pixel accuracy

[WHY]
It was detected in some Freesync HDR tests that displays were not
reaching their maximum nominal brightness.

[HOW]
The Multi-plane combiner (MPC) Output Gamma (OGAM) block builds a
discrete Lookup Table (LUT). When the display's maximum brightness
falls in between two values, having to be linearly interpolated by
the hardware, rounding issues might occur that will cause the
display to never reach its maximum brightness.
The fix involves doing the calculations backwards, ensuring that
the interpolation in the maximum brightness values translates to an
output of 1.0.

Signed-off-by: Felipe Clark <felclark@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>