drm: Use pgprot_writecombine in GEM GTT mapping to get the right bits for !PAT.

Otherwise, the PAGE_CACHE_WC would end up getting us a UC-only mapping, and
the write performance of GTT maps dropped 10x.
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
[anholt: cleaned up unused var]
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/drm_gem.c

@@ -505,7 +505,6 @@ int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma)
 	struct drm_local_map *map = NULL;
 	struct drm_gem_object *obj;
 	struct drm_hash_item *hash;
-	unsigned long prot;
 	int ret = 0;
 
 	mutex_lock(&dev->struct_mutex);
@@ -538,11 +537,7 @@ int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma)
 	vma->vm_ops = obj->dev->driver->gem_vm_ops;
 	vma->vm_private_data = map->handle;
 	/* FIXME: use pgprot_writecombine when available */
-	prot = pgprot_val(vma->vm_page_prot);
-#ifdef CONFIG_X86
-	prot |= _PAGE_CACHE_WC;
-#endif
-	vma->vm_page_prot = __pgprot(prot);
+	vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
 
 	/* Take a ref for this mapping of the object, so that the fault
 	 * handler can dereference the mmap offset's pointer to the object.