drm/amd/display: Fix OGAM LUT calculation precision

[Why]
The OGAM LUT precision was accumulating too much error
in the higher end.

[How]
Instead of calculating all points of the LUT in relation
to the previous ones, perform a full calculation in one
of the intermediate segments to stop error propagation.

Signed-off-by: Felipe Clark <Felipe.Clark@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>