drm/i915/gvt: return error code if dma map iova failed

When doing dma map failed for a pfn, kvmgt should unpin the
pfn and return error code to device module driver

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