drm/amd/display: Use hardware max low point when sampling OTF

The MAX_LOW_POINT macro should reflect the maximum low point within
hardware. Otherwise, sampling for the hardware points from the output
transfer function (OTF) will be incorrect.

Also, fix usage of MAX_LOW_POINT accordingly.

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>