drm/i915/gvt: switch to user vfio_group_pin/upin_pages

substitute vfio_pin_pages() and vfio_unpin_pages() with
vfio_group_pin_pages() and vfio_group_unpin_pages(), so that
it will not go through looking up, checking, referencing,
dereferencing of VFIO group in each call.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200313031151.8042-1-yan.y.zhao@intel.com