drm/i915: shut up gen8+ SDE irq dmesg noise, again

We still keep getting

[    4.249930] [drm:gen8_irq_handler [i915]] *ERROR* The master control interrupt lied (SDE)!

This reverts

commit 820da7ae
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Wed Nov 25 16:47:23 2015 +0200

    Revert "drm/i915: shut up gen8+ SDE irq dmesg noise"

which in itself is a revert, so this is just doing

commit 97e5ed11
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Oct 23 10:56:12 2015 +0200

    drm/i915: shut up gen8+ SDE irq dmesg noise

all over again. I'll stop pretending I understand what's going on like I
did when I thought I'd fixed this for good in

commit 6a39d7c9
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Wed Nov 25 16:47:22 2015 +0200

    drm/i915: fix the SDE irq dmesg warnings properly
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Reference: http://mid.gmane.org/20151213124945.GA5715@nuc-i3427.alporthouse.com
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92084
Cc: drm-intel-fixes@lists.freedesktop.org
Fixes: 820da7ae ("Revert "drm/i915: shut up gen8+ SDE irq dmesg noise"")
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452155350-14658-1-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/i915_irq.c

@@ -2414,9 +2414,13 @@ static irqreturn_t gen8_irq_handler(int irq, void *arg)
 				spt_irq_handler(dev, pch_iir);
 			else
 				cpt_irq_handler(dev, pch_iir);
-		} else
-			DRM_ERROR("The master control interrupt lied (SDE)!\n");
-
+		} else {
+			/*
+			 * Like on previous PCH there seems to be something
+			 * fishy going on with forwarding PCH interrupts.
+			 */
+			DRM_DEBUG_DRIVER("The master control interrupt lied (SDE)!\n");
+		}
 	}
 
 	I915_WRITE_FW(GEN8_MASTER_IRQ, GEN8_MASTER_IRQ_CONTROL);