drm/vmwgfx: Fix race issue calling pin_user_pages

pin_user_pages() is unsafe without protection of mmap_lock,
fix it by calling pin_user_pages_fast().

Fixes: 7a7a933e ("drm/vmwgfx: Introduce VMware mks-guest-stats")
Signed-off-by: Dawei Li <set_pte_at@outlook.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/TYWP286MB23193621CB443E1E1959A00BCA3E9@TYWP286MB2319.JPNP286.PROD.OUTLOOK.COM