modesetting: Enable GAMMA_LUT for lut's with up to 4096 slots.

A lut size of 4096 slots has been verified to work correctly,
as tested with amdgpu-kms. Intel Tigerlake Gen12 hw has a very
large GAMMA_LUT size of 262145 slots, but also issues with its
current GAMMA_LUT implementation, as of Linux 5.14.

Therefore we keep GAMMA_LUT off for large lut's. This currently
excludes Intel Icelake, Tigerlake and later.

This can be overriden via the "UseGammaLUT" boolean xorg.conf option
to force use of GAMMA_LUT on or off.

See following link for the Tigerlake situation:
https://gitlab.freedesktop.org/drm/intel/-/issues/3916#note_1085315



Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
(cherry picked from commit 66e5a5bb)