drm/i915/gvt: Alloc and Init guest opregion at vgpu creation

Currently guest opregion is allocated and initialised when guest
write opregion base register. This is too late for kvmgt, so
move it to vgpu_create time.

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