panfrost: Handle null textures robustly

panfrost: Handle null textures robustly

This is really dumb.

But this fixes arb_shader_language_420pack-active-sampler-conflict on v7 which
otherwise dereferences a null pointer trying to access the nonexistant texture
arrays, or DATA_INVALID_FAULTs if you give it a texture array filled with
zeroes. But it seems happy if you bind in null textures. This is dumb but less
faults in Piglit is good for reducing flakes.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21891>