drm/i915/gvt: allow several MPT methods to be NULL

Hypervisors are different, the MPT ops is a only superset of
all possibly supported hypervisors. There might be other way
out of the MPT to achieve same target. e.g. vfio-based kvmgt
won't provide map_gfn_to_mfn method to establish guest EPT
mapping for aperture, since it will be done in QEMU/KVM, MMIO
is also trapped elsewhere, etc.

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