drm/i915: Avoid GPU hang when coming out of s3 or s4

This patch fixes a timing issue that causes a GPU hang when the system
comes out of power saving.

During pm_resume, We are submitting batchbuffers before enabling
Interrupts this is causing us to miss the context switch interrupt,
and in consequence intel_execlists_handle_ctx_events is not triggered.

This patch is based on a patch from Deepak S <deepak.s@intel.com>
from another platform.

The patch fixes an issue introduced by:
  commit e7778be1eab918274f79603d7c17b3ec8be77386
  drm/i915: Fix startup failure in LRC mode after recent init changes

The above patch added a call to init_context() to fix an issue introduced
by a previous patch. But, it then opened up a small timing window for the
batches being added by the init_context (basically setting up the context)
to complete before the interrupts have been turned on, thus hanging the
GPU.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89600
Cc: stable@vger.kernel.org # 4.0+
Signed-off-by: Peter Antoine <peter.antoine@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[Jani: fixed typo in subject, massaged the comments a bit]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_drv.c

@@ -699,6 +699,16 @@ static int i915_drm_resume(struct drm_device *dev)
 	intel_init_pch_refclk(dev);
 	drm_mode_config_reset(dev);
 
+	/*
+	 * Interrupts have to be enabled before any batches are run. If not the
+	 * GPU will hang. i915_gem_init_hw() will initiate batches to
+	 * update/restore the context.
+	 *
+	 * Modeset enabling in intel_modeset_init_hw() also needs working
+	 * interrupts.
+	 */
+	intel_runtime_pm_enable_interrupts(dev_priv);
+
 	mutex_lock(&dev->struct_mutex);
 	if (i915_gem_init_hw(dev)) {
 		DRM_ERROR("failed to re-initialize GPU, declaring wedged!\n");
@@ -706,9 +716,6 @@ static int i915_drm_resume(struct drm_device *dev)
 	}
 	mutex_unlock(&dev->struct_mutex);
 
-	/* We need working interrupts for modeset enabling ... */
-	intel_runtime_pm_enable_interrupts(dev_priv);
-
 	intel_modeset_init_hw(dev);
 
 	spin_lock_irq(&dev_priv->irq_lock);