Unfortunately gallium doesn't match what the hw wants perfectly here, in
using a separate CSO for each texture/sampler.  So we have to use a hash
table to map the collection of texture/samplers to hw state object.

We probably could use separate hw state objects for texture and sampler
state, but mesa/st tends to update the tex and samp state together.

Signed-off-by: Rob Clark <robdclark@gmail.com>