drm/exynos: fixed exception to page allocation failure

this patch corrects to deallocate the pages allocated already
at alloc_page failure.
Signed-off-by: NInki Dae <inki.dae@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_gem.c

@@ -118,7 +118,7 @@ struct page **exynos_gem_get_pages(struct drm_gem_object *obj,
 	return pages;
 
 fail:
-	while (i--)
+	while (--i)
 		__free_page(pages[i]);
 
 	drm_free_large(pages);