radeonsi: rework uploading border colors

radeonsi: rework uploading border colors

The border colors are uploaded only once when the state is created.

This brings truly immutable sampler descriptors, because they don't have
to be updated every time a sampler state is re-bound.

It also moves the TA_BC_BASE_ADDR registers to init_config, removing one
more state. The catch is there is now a limit: only 4096 border colors can
be used by one context. I don't think that will be a problem.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>