drm/virtio: use virtio_gpu_object->hw_res_handle in virtio_gpu_mode_dumb_create()

Drop pointless resid variable in virtio_gpu_mode_dumb_create(), just use
the hw_res_handle field in virtio_gpu_object directly.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181019061847.18958-4-kraxel@redhat.com