drm/i915/gvt: Only assign ppgtt root at dispatch time

This moves ppgtt root hook out of scan and shadow function,
as it's only required at dispatch time. Also make sure this
checks against shadow mm to be ready, otherwise bail to fail
earlier.

Reviewed-by: Xiong Zhang <xiong.y.zhang@intel.com>
Cc: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>