r600g: do fine-grained sampler state updates

r600g: do fine-grained sampler state updates

Update only those sampler states which are changed in a shader stage,
instead of always updating all sampler states in the shader stage.
That requires keeping a bitmask of those states which are enabled, and those
states which are dirty at a given point (subset of enabled states).

This is similar to how sampler views, constant buffers, and vertex buffers
are handled.

Reviewed-by: Jerome Glisse <jglisse@redhat.com>