drm/i915: Filter out no-op frontbuffer tracking flushes

Paulo noticed that the fbc frontbuffer tracking flush callback
occasionally gets a call without any bit set. This can happen when we
have to filter flush calls due to e.g. gpu rendering. Filter these
out.
Reported-by: Paulo Zanoni <przanoni@gmail.com>
Cc: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/i915/intel_frontbuffer.c

@@ -175,6 +175,9 @@ void intel_frontbuffer_flush(struct drm_device *dev,
 	frontbuffer_bits &= ~dev_priv->fb_tracking.busy_bits;
 	mutex_unlock(&dev_priv->fb_tracking.lock);
 
+	if (!frontbuffer_bits)
+		return;
+
 	intel_mark_fb_busy(dev, frontbuffer_bits);
 
 	intel_edp_drrs_flush(dev, frontbuffer_bits);