WIP: modesetting: Use atomic more atomically

drmmode_set_desired_modes (reachable from CreateScreenResources, EnterVT, etc.) currently works by doing ->set_mode_major on each CRTC. This is silly, as atomic lets us configure every CRTC at once. It's also fragile, because we're not trying very hard to ensure the intermediate states are valid. It's also ugly, VT switches will be way more flickery than they need to be.

This series works for initial setup and VT switch, but not the actual "resize" and "rotate" parts of RANDR yet.