d rm/i915: freeze display before the interrupts and GT

Since we started using intel_runtime_pm_disable_interrupts() at normal
(non-runtime) suspend/resume, we had to remove a WARN from
ironlake_disable_display_irq to avoid a case where we were doing the
correct thing and the WARN was not really needed. The problem is that
the WARN was useful in other cases, and its removal can hide some bugs
that we would catch automatically.

To be able to add back the WARN, we have to call intel_crtc_control()
before interrupts are disabled, which is what this patch currently
does.

Also notice that Ville's patch from the Watermarks series "drm/i915:
Leave interrupts enabled while disabling crtcs during suspend" also
did a change that's equivalent to the one we're doing on this patch,
with the exception that its original patch, when applied to the
current tree, procduces a WARN.

Related commits:

commit daa390e5ee45cc051d6bf37b296901f2f92b002d
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
    drm/i915: don't warn if IRQs are disabled when shutting down display IRQs

commit e11aa36230
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
    drm/i915: use runtime irq suspend/resume in freeze/thaw

Note that the function part of this patch has already been done in

commit 0e32b39cee
Author: Dave Airlie <airlied@redhat.com>
Date:   Fri May 2 14:02:48 2014 +1000

    drm/i915: add DP 1.2 MST support (v0.7)

with the fixup

commit 09b64267c1
Author: Dave Airlie <airlied@redhat.com>
Date:   Wed Jul 23 14:25:24 2014 +1000

    drm/i915: don't suspend gt until after we disable irqs and display (v2)

so all that's left from Paulo's patch is reinstating the WARNING.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Explain conflict resolution with Dave's DP MST patches with a
note in the commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -151,7 +151,7 @@ ironlake_disable_display_irq(struct drm_i915_private *dev_priv, u32 mask)
 {
 	assert_spin_locked(&dev_priv->irq_lock);
 
-	if (!intel_irqs_enabled(dev_priv))
+	if (WARN_ON(!intel_irqs_enabled(dev_priv)))
 		return;
 
 	if ((dev_priv->irq_mask & mask) != mask) {