drm/i915: Never fully mask the the EI up rps interrupt on SNB/IVB

SNB (and IVB too I suppose) starts to misbehave if the GPU gets stuck
in an infinite batch buffer loop. The GPU apparently hogs something
critical and CPUs start to lose interrupts and whatnot. We can keep
the system limping along by unmasking some interrupts in
GEN6_PMINTRMSK. The EI up interrupt has been previously chosen for
that task, so let's never mask it.

v2: s/gen6_rps_pm_mask/gen6_sanitize_rps_pm_mask/ (Chris)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93122
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1464014568-4529-1-git-send-email-ville.syrjala@linux.intel.com
Cc: stable@vger.kernel.org

drivers/gpu/drm/i915/intel_pm.c

@@ -4892,7 +4892,8 @@ void gen6_rps_idle(struct drm_i915_private *dev_priv)
 		else
 			gen6_set_rps(dev_priv, dev_priv->rps.idle_freq);
 		dev_priv->rps.last_adj = 0;
-		I915_WRITE(GEN6_PMINTRMSK, 0xffffffff);
+		I915_WRITE(GEN6_PMINTRMSK,
+			   gen6_sanitize_rps_pm_mask(dev_priv, ~0));
 	}
 	mutex_unlock(&dev_priv->rps.hw_lock);