Detach CRTC data from drm_output

Hello! In this series we move the CRTC data from struct drm_output to the new struct drm_crtc.

This allow us to get rid of the array unused_crtcs, as we can do the following check in order to see if a CRTC is not in use by any output:

if (crtc->output)
    // CRTC in use
else
    // CRTC not in use

Besides making the code shorter, it also makes it clearer.