drm/i915: i915_gem_object_finish_gtt must always release gtt mmap

Even if the object is no longer in the GTT domain, there may still be
a user space mapping which needs to be released.

Without this fix, render-based text (mostly in firefox) would
occasionally get corrupted when the system was under load.
Signed-off-by: Keith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -2155,15 +2155,15 @@ static void i915_gem_object_finish_gtt(struct drm_i915_gem_object *obj)
 {
 	u32 old_write_domain, old_read_domains;
 
-	if ((obj->base.read_domains & I915_GEM_DOMAIN_GTT) == 0)
-		return;
-
 	/* Act a barrier for all accesses through the GTT */
 	mb();
 
 	/* Force a pagefault for domain tracking on next user access */
 	i915_gem_release_mmap(obj);
 
+	if ((obj->base.read_domains & I915_GEM_DOMAIN_GTT) == 0)
+		return;
+
 	old_read_domains = obj->base.read_domains;
 	old_write_domain = obj->base.write_domain;