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: drm/intel#3916 (comment 1085315) Signed-off-by: Mario Kleiner <firstname.lastname@example.org>