drm/i915/gvt: free VFIO region space in vgpu detach

VFIO region space is allocated when one region is registered for
one vgpu. So free the space when destroy the vgpu.

Also change the parameter of detach_vgpu callback to use vgpu directly.

Fixes: b851adea ("drm/i915/gvt: Add opregion support")
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Hang Yuan <hang.yuan@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>