drm/ast: Delete an unnecessary check before drm_gem_object_unreference_unlocked()

The drm_gem_object_unreference_unlocked() function tests whether
its argument is NULL and then returns immediately.
Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/f28c063f-ec56-e62b-9370-89ce833fa4e5@users.sourceforge.net

drivers/gpu/drm/ast/ast_main.c

@@ -295,9 +295,8 @@ static int ast_get_dram_info(struct drm_device *dev)
 static void ast_user_framebuffer_destroy(struct drm_framebuffer *fb)
 {
 	struct ast_framebuffer *ast_fb = to_ast_framebuffer(fb);
-	if (ast_fb->obj)
-		drm_gem_object_unreference_unlocked(ast_fb->obj);
 
+	drm_gem_object_unreference_unlocked(ast_fb->obj);
 	drm_framebuffer_cleanup(fb);
 	kfree(fb);
 }