drm/vmwgfx: Replace "vmw_num_pages" with "PFN_UP"

we counld use PFN_UP instead of vmw_num_pages()

Signed-off-by: Cai Huoqing <caihuoqing@baidu.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210802033552.990-1-caihuoqing@baidu.com