softpipe/draw/tgsi: simplify driver/tgsi sampler interface

softpipe/draw/tgsi: simplify driver/tgsi sampler interface

Use a single sampler adapter instead of per-sampler-unit samplers,
and just pass along texture unit and sampler unit in the calls.
The reason is that for dx10-style sample opcodes pre-wired
samplers including all the texture state aren't really feasible (and for
sample_i/sviewinfo we don't even have samplers).
Of course right now softpipe doesn't actually do anything more than
just look up all its pre-wired per-texunit/per-samplerunit sampler as
it did before so this doesn't really achieve much except one more
function call, however this is now all softpipe's fault (fixing that in
a way which doesn't suck is still an unsolved problem).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>