`mode::get_gamma` and `set_gamma` should check lengths of slices against `size` argument

[jimblandy]

The functions mode::get_gamma and mode::set_gamma are not marked unsafe, but the kernel will read/write as many entries of the red, green, and blue slices as directed by crtc->gamma_size regardless of their actual lengths.

It seems to me that either these functions should be marked unsafe, or they should dynamically check that the slices' lengths are adequate for the given size. The kernel doesn't perform any accesses if the size passed in does not match the CRTC's actual gamma_size.