i915/gvt: Move gvt mapping cache initialization to intel_vgpu_init_dev()

i915/gvt: Move gvt mapping cache initialization to intel_vgpu_init_dev()

vfio container registers .dma_unmap() callback after the device is opened.
So it's fine for mdev drivers to initialize internal mapping cache in
.open_device(). See vfio_device_container_register().

Now with iommufd an access ops with an unmap callback is registered when
the device is bound to iommufd which is before .open_device() is
called. This implies gvt's .dma_unmap() could be called before its
internal mapping cache is initialized.

The fix is moving gvt mapping cache initialization to vGPU init. While at
it also move ptable initialization together.

Link: https://lore.kernel.org/r/20221202135402.756470-2-yi.l.liu@intel.com
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>