drm/i915: avoid deadlock on failure paths in __intel_framebuffer_create()

Since a8bb6818 __intel_framebuffer_create() is called
with struct_mutex held, so it should use drm_gem_object_unreference()
instead of drm_gem_object_unreference_unlocked().

Found by Linux Driver Verification project (linuxtesting.org).

This regression has been introduced in

commit a8bb6818
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Feb 10 18:00:39 2014 +0100

    drm/i915: Fix error path leak in fbdev fb allocation
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -8421,7 +8421,7 @@ __intel_framebuffer_create(struct drm_device *dev,
 
 	intel_fb = kzalloc(sizeof(*intel_fb), GFP_KERNEL);
 	if (!intel_fb) {
-		drm_gem_object_unreference_unlocked(&obj->base);
+		drm_gem_object_unreference(&obj->base);
 		return ERR_PTR(-ENOMEM);
 	}
 
@@ -8431,7 +8431,7 @@ __intel_framebuffer_create(struct drm_device *dev,
 
 	return &intel_fb->base;
 err:
-	drm_gem_object_unreference_unlocked(&obj->base);
+	drm_gem_object_unreference(&obj->base);
 	kfree(intel_fb);
 
 	return ERR_PTR(ret);