radeonsi: move sampler descriptors from IB to memory

radeonsi: move sampler descriptors from IB to memory

Sampler descriptors are now represented by si_descriptors.
This also adds support for fine-grained sampler state updates and
the border color update is now isolated in a separate function.

Border colors have been broken if texturing from multiple shader stages is
used. This patch doesn't change that.

BTW, blitting already makes use of fine-grained state updates.
u_blitter uses 2 textures at most, so we only have to save 2.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>