gallium: Pack the atomic counters just above the SSBOs.

We carve out half the SSBO space for atomics, and we were just binding
them way up there.  freedreno was then using a remapping table to map the
sparse buffer index back down, since space in the descriptor array is a
shared resource that may limit parallelism.  That remapping table
generated inside of the ir3 compiler is getting thoroughly in the way of
implementing vulkan descriptor sets.

We will be able to get rid of the freedreno's remapping table, and
hopefully save shared resources on other hardware, by packing the atomics
tightly above the SSBOs (like i965 does).  We already rebind the shader
buffers on program change if either the old or new program has SSBOs or
ABOs, so this doesn't necessarily increase the program state change cost
(the only cost increase I can come up with is if you're using the same
atomic counter without rebinding it across changes of programs with
varying SSBO counts, meaning it would now bounce around index space).

Reviewed-by: Marek Olšák <[email protected]>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3240>

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