drm/i915: only recompress FBC after flushing a drawing operation

There's no need to stop and restart FBC, which is quite expensive as
we have to revalidate the CRTC state. After flushing a drawing
operation we know the CRTC state hasn't changed, so a nuke
(recompress) should be fine.

v2: Make it simpler (Chris).
v3: Rewrite the patch again due to patch order changes.
v4: Rewrite commit message (Chris).
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/

drivers/gpu/drm/i915/intel_fbc.c

@@ -935,8 +935,12 @@ void intel_fbc_flush(struct drm_i915_private *dev_priv,
 	dev_priv->fbc.busy_bits &= ~frontbuffer_bits;
 
 	if (!dev_priv->fbc.busy_bits && dev_priv->fbc.enabled) {
-		__intel_fbc_deactivate(dev_priv);
-		__intel_fbc_update(dev_priv->fbc.crtc);
+		if (origin != ORIGIN_FLIP && dev_priv->fbc.active) {
+			intel_fbc_recompress(dev_priv);
+		} else {
+			__intel_fbc_deactivate(dev_priv);
+			__intel_fbc_update(dev_priv->fbc.crtc);
+		}
 	}
 
 	mutex_unlock(&dev_priv->fbc.lock);