mesa/st: add support for dynamic sampler offsets

mesa/st: add support for dynamic sampler offsets

Replace the plain sampler index with a register reference to a sampler.
We also need to keep track of the sampler array size when there is a
relative reference so that we can mark the whole array used.

To facilitate implementation, we add a separate ADDR register that
exclusively handles the sampler relative address. Other approaches would
be more invasive.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>