drm/i915: Move drm_framebuffer_unreference out of struct_mutex for takeover

intel_user_framebuffer_destroy() requires the struct_mutex for its
object bookkeeping, so this means that all calls to
drm_framebuffer_unreference must be held without that lock.

This is a simplified version of the identically named patch by Chris Wilson.

    Regression from commit ab8d6675
    Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
    Date:   Mon Feb 2 15:44:15 2015 +0000

        drm/i915: Track old framebuffer instead of object

v2: Bikeshedding.

References: https://bugs.freedesktop.org/show_bug.cgi?id=89166
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -14259,6 +14259,7 @@ void intel_modeset_gem_init(struct drm_device *dev)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct drm_crtc *c;
 	struct drm_i915_gem_object *obj;
+	int ret;
 
 	mutex_lock(&dev->struct_mutex);
 	intel_init_gt_powersave(dev);
@@ -14283,16 +14284,18 @@ void intel_modeset_gem_init(struct drm_device *dev)
 	 * pinned & fenced.  When we do the allocation it's too early
 	 * for this.
 	 */
-	mutex_lock(&dev->struct_mutex);
 	for_each_crtc(dev, c) {
 		obj = intel_fb_obj(c->primary->fb);
 		if (obj == NULL)
 			continue;
 
-		if (intel_pin_and_fence_fb_obj(c->primary,
-					       c->primary->fb,
-					       c->primary->state,
-					       NULL)) {
+		mutex_lock(&dev->struct_mutex);
+		ret = intel_pin_and_fence_fb_obj(c->primary,
+						 c->primary->fb,
+						 c->primary->state,
+						 NULL);
+		mutex_unlock(&dev->struct_mutex);
+		if (ret) {
 			DRM_ERROR("failed to pin boot fb on pipe %d\n",
 				  to_intel_crtc(c)->pipe);
 			drm_framebuffer_unreference(c->primary->fb);
@@ -14300,7 +14303,6 @@ void intel_modeset_gem_init(struct drm_device *dev)
 			update_state_fb(c->primary);
 		}
 	}
-	mutex_unlock(&dev->struct_mutex);
 
 	intel_backlight_register(dev);
 }