drm/tegra: gem: Check before freeing CMA memory

dma_free_writecombine() must not be called on a buffer that couldn't be
allocated. Check for a valid virtual address before attempting to free
the memory to avoid a crash.
Reported-by: NSean Paul <seanpaul@chromium.org>
Reviewed-by: NSean Paul <seanpaul@chromium.org>
Signed-off-by: NThierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/gem.c

@@ -210,7 +210,7 @@ static void tegra_bo_free(struct drm_device *drm, struct tegra_bo *bo)
 		drm_gem_put_pages(&bo->gem, bo->pages, true, true);
 		sg_free_table(bo->sgt);
 		kfree(bo->sgt);
-	} else {
+	} else if (bo->vaddr) {
 		dma_free_writecombine(drm->dev, bo->gem.size, bo->vaddr,
 				      bo->paddr);
 	}