backend/drm: update wlr_drm_connnector.crtc in drm_connector_commit_state()
If the commit fails, then our local state becomes out-of-sync with the kernel's. Additionally, when disabling a connector without going through dealloc_crtc(), conn->crtc would still be set.
Fix this by updating conn->crtc in drm_connector_commit_state().