drm/i915/gvt: enable IOMMU for gvt

gvt driver has a check which doesn't allow to use gvt when host
gpu iommu is enabled. This check can be removed after enable the
iommu support in gvt

Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>