drm/ast: Free container instead of member in ast_user_framebuffer_destroy()

Technically freeing ast_fb->base is the same as freeing ast_fb as 'base'
the first member of the data structure.
Still this makes it cleaner.
Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ast/ast_main.c

@@ -389,7 +389,7 @@ static void ast_user_framebuffer_destroy(struct drm_framebuffer *fb)
 
 	drm_gem_object_unreference_unlocked(ast_fb->obj);
 	drm_framebuffer_cleanup(fb);
-	kfree(fb);
+	kfree(ast_fb);
 }
 
 static const struct drm_framebuffer_funcs ast_fb_funcs = {