modetest: Call dirty fb on modeset

Signed-off-by: Jakob Bornecrantz <jakob@vmware.com>
diff --git a/tests/modetest/modetest.c b/tests/modetest/modetest.c
index f65a033..229ab8a 100644
--- a/tests/modetest/modetest.c
+++ b/tests/modetest/modetest.c
@@ -583,6 +583,10 @@
 
 		ret = drmModeSetCrtc(fd, c[i].crtc, fb_id, x, 0,
 				     &c[i].id, 1, c[i].mode);
+
+		/* XXX: Actually check if this is needed */
+		drmModeDirtyFB(fd, fb_id, NULL, 0);
+
 		x += c[i].mode->hdisplay;
 
 		if (ret) {