drm/i915: drop the struct_mutex when wedged or trying to reset

We grab the struct_mutex in intel_crtc_page_flip, but if we are wedged
or a reset is in progress we bail early but never seem to actually
release the lock.

Fixes: 7f1847e ("drm/i915: Simplify checking of GPU reset_counter in display pageflips")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161128103648.9235-1-matthew.auld@intel.com
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: <stable@vger.kernel.org> # v4.7+
(cherry picked from commit ddbb271)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Author: matt-auld

drivers/gpu/drm/i915/intel_display.c

@@ -12260,7 +12260,7 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 	intel_crtc->reset_count = i915_reset_count(&dev_priv->gpu_error);
 	if (i915_reset_in_progress_or_wedged(&dev_priv->gpu_error)) {
 		ret = -EIO;
-		goto cleanup;
+		goto unlock;
 	}
 
 	atomic_inc(&intel_crtc->unpin_work_count);
@@ -12352,6 +12352,7 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 	intel_unpin_fb_obj(fb, crtc->primary->state->rotation);
 cleanup_pending:
 	atomic_dec(&intel_crtc->unpin_work_count);
+unlock:
 	mutex_unlock(&dev->struct_mutex);
 cleanup:
 	crtc->primary->fb = old_fb;