drm: silence pointless vblank warning.

Some applications/hardware combinations are triggering the message "failed to
acquire vblank counter" to be issued up to 20 times a second, which makes it
both useless and dangerous, as this may hide other important messages.
This changes makes it only appear when people are debugging.
Signed-off-by: Paul Rolland <rol@as2917.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Lost-twice-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_irq.c

@@ -566,7 +566,7 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 
 	ret = drm_vblank_get(dev, crtc);
 	if (ret) {
-		DRM_ERROR("failed to acquire vblank counter, %d\n", ret);
+		DRM_DEBUG("failed to acquire vblank counter, %d\n", ret);
 		return ret;
 	}
 	seq = drm_vblank_count(dev, crtc);