drm/msm/mdp4: cure for the cursor blues (v2)

The hw cursor is relatively adept at triggering underflows, which
manifest as a "blue flash" (since blue is configured as the underflow
color).  Juggle a few things around to tighten up the timing for setting
cursor registers in DONE irq.

And most importantly, don't ever disable the hw cursor.  Instead flip it
to a blank/empty cursor.  This seems far more reliable, as even simply
clearing the cursor-enable bit (with no other updates in previous/
following frames) can in some cases cause underflow.

v1: original
v2: add missing locking spotted by Micah

Cc: Micah Richert <richert@braincorporation.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/mdp/mdp5/mdp5_irq.c

@@ -71,11 +71,11 @@ static void mdp5_irq_mdp(struct mdp_kms *mdp_kms)
 
 	VERB("status=%08x", status);
 
+	mdp_dispatch_irqs(mdp_kms, status);
+
 	for (id = 0; id < priv->num_crtcs; id++)
 		if (status & mdp5_crtc_vblank(priv->crtcs[id]))
 			drm_handle_vblank(dev, id);
-
-	mdp_dispatch_irqs(mdp_kms, status);
 }
 
 irqreturn_t mdp5_irq(struct msm_kms *kms)