etnaviv: handle NULL views in set_sampler_views

etnaviv: handle NULL views in set_sampler_views

Passing NULL for the views parameter should be the same as passing an
array of NULL, according to the documentation. So let's respect that
detail.

This fixes a crash when using GALLIUM_HUD. The wrong handling of views
parameter was causing problems starting with
2813688f8db ("gallium/hud: don't use cso_context to restore VBs, constbuf 0 and sampler views").

Cc: <mesa-stable@lists.freedesktop.org>
Fixes: c9e8b49b885 ("etnaviv: gallium driver for Vivante GPUs")
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8670>