drm/ast: Remove unecessary NULL check in gem_free

The ->gem_free_object never gets called with a NULL pointer, the check
is redundant. Also checking after the upcast allows compilers to elide
it anyway.

Spotted by coverity.

v2: Fix patch subject.
Reviewed-by: NDavid Herrmann <dh.herrmann@gmail.com>
Reviewed-by: NIan Romanick <ian.d.romanick@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/ast/ast_main.c

@@ -418,8 +418,6 @@ void ast_gem_free_object(struct drm_gem_object *obj)
 {
 	struct ast_bo *ast_bo = gem_to_ast_bo(obj);
 
-	if (!ast_bo)
-		return;
 	ast_bo_unref(&ast_bo);
 }