Fix night light without GAMMA_LUT property
The GAMMA_LUT property doesn't necessarily exist when using drivers that doesn't support atomic mode setting, and the way it worked will break night light and other gamma related features. This makes things use the gamma length; if it is higher than 0, it definitely supports it one way or the other, i.e. GAMMA_LUT with the atomic backend, and drmModeCrtcSetGamma() with the legacy/simple backend.
Fixes: 364572b9