st/mesa: remove sampler associated with buffer texture in pbo logic

st/mesa: remove sampler associated with buffer texture in pbo logic

A long time ago, when this was first implemented, not having a sampler
bound would cause problems on Fermi. I didn't work out the reasons, but
the solution was simple -- just put the samplers back in.

Since then, regular texturing paths appear to have lost their associated
samplers which required a fuller investigation and fix in nouveau. Now
that this is done, this code should no longer need a sampler state for
fetching texels from a buffer texture.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>