If drmModeSetCrtc() is called with no fb, mode or connectors for some
CRTC it may still fail, and we should handle that gracefully instead of
assuming it failed to set a non-disabled state.
Closes https://gitlab.gnome.org/GNOME/mutter/issues/70
(cherry picked from commit 6e953e2725d5d5b10d14c7bd479bd99f6853addc)