drm/virtio: Add prepare and cleanup routines for imported dmabuf obj

When an imported dmabuf obj is used as part of an atomic commit, we
need to pin it as part of prepare and unpin it during cleanup of
the associated FB, to make sure that it does not move until the
commit is completed (and also while it is being used on the Host).

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Cc: Chia-I Wu <olvaffe@gmail.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241126031643.3490496-6-vivek.kasireddy@intel.com