drm/amd/display: Increase precision for backlight curve

[Why]
We are currently losing precision when we convert from
16 bit --> 8 bit --> 16 bit.

[How]
We shouldn't down convert unnecessarily and lose precision.
Keep values at 16 bit and use directly.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>