drm/amd/display: Fix regamma not affecting full-intensity color values

Hardware understands the regamma LUT as a piecewise linear function,
with points spaced exponentially along the range. We previously
programmed the LUT for range [2^-10, 2^0). This causes (normalized)
color values of 1 (=2^0) to miss the programmed LUT, and fall onto the
end region.

For DCE, the end region is extrapolated using a single (base, slope)
pair, using the max y-value from the last point in the curve as base.
This presents a problem, since this value affects all three color
channels. Scaling down the intensity of say - the blue regamma curve -
will not affect it's end region. This is especially noticiable when
using RedShift. It scales down the blue and green channels, but leaves
full-intensity colors unshifted.

Therefore, extend the range to cover [2^-10, 2^1) by programming another
hardware segment, containing only one point. That way, we won't be
hitting the end region.

Note that things are a bit different for DCN, since the end region can
be set per-channel.

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>