drm/exynos: fix missed calling of drm_prime_gem_destroy()

When obj->import_attach is existed, code calling drm_prime_gem_destroy()
was removed from commit 67e93c808b48 ("drm/exynos: stop copying sg
table"), and it's a fault.

The drm_prime_gem_destroy() is cleanup function which GEM drivers need
to call when they use drm_gem_prime_import() to import dma-bufs, so
exynos-drm driver using drm_gem_prime_import() needs calling
drm_prime_gem_destroy().

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
diff --git a/drivers/gpu/drm/exynos/exynos_drm_gem.h b/drivers/gpu/drm/exynos/exynos_drm_gem.h
index cd62f84..97bb48b 100644
--- a/drivers/gpu/drm/exynos/exynos_drm_gem.h
+++ b/drivers/gpu/drm/exynos/exynos_drm_gem.h
@@ -39,6 +39,7 @@
  *	- this address could be physical address without IOMMU and
  *	device address with IOMMU.
  * @pages: Array of backing pages.
+ * @sgt: Imported sg_table.
  *
  * P.S. this object would be transferred to user as kms_bo.handle so
  *	user can access the buffer through kms_bo.handle.
@@ -52,6 +53,7 @@
 	dma_addr_t		dma_addr;
 	struct dma_attrs	dma_attrs;
 	struct page		**pages;
+	struct sg_table		*sgt;
 };
 
 struct page **exynos_gem_get_pages(struct drm_gem_object *obj, gfp_t gfpmask);