drm/virtio: move virtio_gpu_mem_entry initialization to new function

Introduce new virtio_gpu_object_shmem_init() helper function which will
create the virtio_gpu_mem_entry array, containing the backing storage
information for the host.  For the most path this just moves code from
virtio_gpu_object_attach().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200207074638.26386-5-kraxel@redhat.com


Reviewed-by: Chia-I Wu <olvaffe@gmail.com>