backend/drm: ensure CRTC is set up in drm_connector_state_init()
In 1 we discovered a bug where wlr_drm_connector_state.primary_fb would not be set up correctly because drm_connector_alloc_crtc() was called after drm_connector_state_init(). This is tricky to discover, so instead assert() that we have a usable CRTC by the time drm_connector_state_init() is called.