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

We get tons of cases where the master interrupt handler apparently set
a bit, with the SDEIIR disagreeing. No idea what's going on there, but
it's consistent on gen8+, no one seems to care about it and it's
making CI results flaky.

Shut it up.

No idea what's going on here, but we've had fun with PCH interrupts
before:

commit 44498aea29
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Fri Feb 22 17:05:28 2013 -0300

    drm/i915: also disable south interrupts when handling them

Note that there's a regression report in Bugzilla, and other
regression reports on the mailing lists keep croping up. But no ill
effects have ever been reported. But for paranoia still keep the
message at a debug level as a breadcrumb, just in case.

This message was introduced in

commit 38cc46d73e
Author: Oscar Mateo <oscar.mateo@intel.com>
Date:   Mon Jun 16 16:10:59 2014 +0100

    drm/i915/bdw: Ack interrupts before handling them (GEN8)

v2: Improve commit message a bit.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445590572-23631-2-git-send-email-daniel.vetter@ffwll.ch
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92084
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80896
Acked-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -2353,9 +2353,13 @@ 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);