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.

[1]: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4569

backend/drm/drm.c

@@ -544,16 +544,20 @@ static void drm_connector_state_init(struct wlr_drm_connector_state *state,
 		state->mode.type = DRM_MODE_TYPE_USERDEF;
 	}
 
-	if (conn->crtc != NULL) {
+	if (output_pending_enabled(&conn->output, base)) {
+		// The CRTC must be set up before this function is called
+		assert(conn->crtc != NULL);
+
 		struct wlr_drm_plane *primary = conn->crtc->primary;
 		if (primary->queued_fb != NULL) {
 			state->primary_fb = drm_fb_lock(primary->queued_fb);
 		} else if (primary->current_fb != NULL) {
 			state->primary_fb = drm_fb_lock(primary->current_fb);
 		}
-	}
-	if (conn->crtc != NULL && conn->cursor_enabled) {
+
+		if (conn->cursor_enabled) {
 			struct wlr_drm_plane *cursor = conn->crtc->cursor;
+			assert(cursor != NULL);
 			if (conn->cursor_pending_fb != NULL) {
 				state->cursor_fb = drm_fb_lock(conn->cursor_pending_fb);
 			} else if (cursor->queued_fb != NULL) {
@@ -562,6 +566,7 @@ static void drm_connector_state_init(struct wlr_drm_connector_state *state,
 				state->cursor_fb = drm_fb_lock(cursor->current_fb);
 			}
 		}
+	}
 }
 
 static void drm_connector_state_finish(struct wlr_drm_connector_state *state) {