drm/virtio: batch plane updates (pageflip)

Move virtio_gpu_notify() to higher-level functions for
virtio_gpu_cmd_resource_flush(), virtio_gpu_cmd_set_scanout() and
virtio_gpu_cmd_transfer_to_host_{2d,3d}().

virtio_gpu_primary_plane_update() will notify only once for a series
of commands (restores plane update command batching).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200214125535.26349-4-kraxel@redhat.com