drm/kms: use drm_atomic_helper_framebuffer_changed when waiting for vblanks

Waiting for vblanks takes too long on our boards. Reverting to the old API
which was calling drm_atomic_helper_framebuffer_changed.

With this, we can achieve the framerate in kernel 4.9.

Tested-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Tested-by: Razvan Stefanescu <razvan.stefanescu@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>

Author: ehristev

drivers/gpu/drm/drm_atomic_helper.c

@@ -1400,6 +1400,9 @@ drm_atomic_helper_wait_for_vblanks(struct drm_device *dev,
 		if (!new_crtc_state->active)
 			continue;
 
+		if (!drm_atomic_helper_framebuffer_changed(dev,
+                                old_state, crtc))
+			continue;
 		ret = drm_crtc_vblank_get(crtc);
 		if (ret != 0)
 			continue;