etnaviv: Skip 'empty' cmd streams

etnaviv: Skip 'empty' cmd streams

Make use of etna_cmd_stream_mark_end_of_context_init(..) to mark the
end of context init states. If the cmd stream is untouched until
etna_cmd_stream_flush(..) gets called the drm layer will downgrade this
specific cmd stream to a noop.

This safes us from calling into the kernel with an 'empty' cmd stream.

Signed-off-by: Christian Gmeiner <cgmeiner@igalia.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25849>