radeonsi: avoid crash when a sampler state is bound for a buffer texture

radeonsi: avoid crash when a sampler state is bound for a buffer texture

Sampler states don't really make sense with buffer textures, but they
can be set anyway, so we need to be defensive here. This bug was lurking
for a while and was finally noticed due to PBO uploads setting sampler
states.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94284
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Laurent Carlier <lordheavym@gmail.com>
Tested-by: Shawn Starr <shawn.starr@rogers.com>