backend/drm: move legacy-specific checks to legacy.c

Now that we have a test_only arg in crtc_commit, we can move the
legacy checks to legacy.c.

backend/drm/drm.c

@@ -419,14 +419,6 @@ static bool drm_connector_set_pending_fb(struct wlr_drm_connector *conn,
 		}
 		break;
 	case WLR_OUTPUT_STATE_BUFFER_SCANOUT:;
-		/* Legacy never gets to have nice things. But I doubt this would ever work,
-		 * and there is no reliable way to try, without risking messing up the
-		 * modesetting state. */
-		if (drm->iface == &legacy_iface) {
-			wlr_drm_conn_log(conn, WLR_DEBUG,
-				"Cannot use direct scan-out with legacy KMS API");
-			return false;
-		}
 		if (!drm_fb_import(&plane->pending_fb, drm, state->buffer,
 				&crtc->primary->formats)) {
 			wlr_drm_conn_log(conn, WLR_DEBUG,

backend/drm/legacy.c

@@ -8,9 +8,24 @@
 #include "backend/drm/iface.h"
 #include "backend/drm/util.h"
 
+static bool legacy_crtc_test(struct wlr_drm_connector *conn,
+		const struct wlr_output_state *state) {
+	if ((state->committed & WLR_OUTPUT_STATE_BUFFER) &&
+			state->buffer_type == WLR_OUTPUT_STATE_BUFFER_SCANOUT) {
+		wlr_drm_conn_log(conn, WLR_DEBUG,
+			"Cannot use direct scan-out with legacy KMS API");
+		return false;
+	}
+
+	return true;
+}
+
 static bool legacy_crtc_commit(struct wlr_drm_backend *drm,
 		struct wlr_drm_connector *conn, const struct wlr_output_state *state,
 		uint32_t flags, bool test_only) {
+	if (!legacy_crtc_test(conn, state)) {
+		return false;
+	}
 	if (test_only) {
 		return true;
 	}