drm/i915/gvt: Let each vgpu has separate opregion memory

Currently every vgpu share a common gvt opregion memory, but
it is freed at vgpu destroy, then the later vgpu doesn't have
opregion memory once the first vgpu is destroyed. This cause
guest function failure like reboot, second or later boot.

This patch allocate and init virt opregion memory for each
vgpu, so this memory could be freed at vgpu destroy.

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