softpipe: don't use samplers with prebaked sampler and sampler_view state

softpipe: don't use samplers with prebaked sampler and sampler_view state

This is needed for handling the dx10-style sample opcodes.
This also simplifies the logic by getting rid of sampler variants
completely (sampler_views though OTOH have sort of variants because
some of their state is different depending on the shader stage they
are bound to).
No significant performance difference (openarena run:
840 frames in 459.8 seconds vs. 840 frames in 460.5 seconds).

v2: fix reference counting bug spotted by Jose.

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