Call drmmode_crtc_gamma_do_set from drmmode_setup_colormap

Instead of from drmmode_set_mode_major. There's no need to re-set the
gamma LUT on every modeset, the kernel should preserve it.

(Ported from amdgpu commit baea4fa492f635cdfe746a84be2e337d9aeae8a9)
Acked-by: Alex Deucher <alexander.deucher@amd.com>