drm/i915/gvt/kvmgt: read/write GPA via KVM API

Previously to read/write a GPA, we at first try to pin the GFN it belongs
to, then translate the pinned PFN to a kernel HVA, then read/write it.
This is however not necessary. A GFN should be pinned IFF it would be
accessed by peripheral devices (DMA), not by CPU. This patch changes
the read/write method to KVM API, which will leverage userspace HVA
and copy_{from|to}_usr instead.

Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>