drm/rockchip: Cocci spatch "vma_pages"

Use vma_pages function on vma object instead of explicit computation.
Found by coccinelle spatch "api/vma_pages.cocci"
Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1505946334393-988165015-7-diffsplit-thomas@m3y3r.de

drivers/gpu/drm/rockchip/rockchip_drm_gem.c

@@ -220,7 +220,7 @@ static int rockchip_drm_gem_object_mmap_iommu(struct drm_gem_object *obj,
 {
 	struct rockchip_gem_object *rk_obj = to_rockchip_obj(obj);
 	unsigned int i, count = obj->size >> PAGE_SHIFT;
-	unsigned long user_count = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+	unsigned long user_count = vma_pages(vma);
 	unsigned long uaddr = vma->vm_start;
 	unsigned long offset = vma->vm_pgoff;
 	unsigned long end = user_count + offset;