drm/shmem: Put pages independent of a SG table being set

If a driver does its own management of pages, the shmem helper object's
pages array could be allocated when a SG table is not. There's not
really any  good reason to tie putting pages with having a SG table when
freeing the object, so just put pages if the pages array is populated.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808222200.13176-3-robh@kernel.org

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -119,11 +119,11 @@ void drm_gem_shmem_free_object(struct drm_gem_object *obj)
 		if (shmem->sgt) {
 			dma_unmap_sg(obj->dev->dev, shmem->sgt->sgl,
 				     shmem->sgt->nents, DMA_BIDIRECTIONAL);
-
-			drm_gem_shmem_put_pages(shmem);
 			sg_free_table(shmem->sgt);
 			kfree(shmem->sgt);
 		}
+		if (shmem->pages)
+			drm_gem_shmem_put_pages(shmem);
 	}
 
 	WARN_ON(shmem->pages_use_count);