etnaviv: reinstate accidentially commented assert

I added this hack to my tree when testing another MR and ended up
squashing it into c2a3236d1a (etnaviv: clean up tiling setup in
etna_compile_rs_state) by accident when doing some changes to this
commit. Reinstate the assert.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16212>