UPSTREAM: drm/virtio: remove prime pin/unpin callbacks.

virtio-gpu objects never move around, so effectively they are
pinned all the time.  Therefore we don't need the (optional)
pin/unpin callbacks.  Remove them.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190227144441.6755-3-kraxel@redhat.com


(cherry picked from commit 873f51d43439d554f7299e88edfb208924bef6eb)
Signed-off-by: Greg Hartman <ghartman@google.com>
BUG: 139386237
Change-Id: Ic20a17cdaef7a4764c98c47e5afbe9d98766c93a