i965: Use nir_lower_atomics_to_ssbos and delete ABO compiler code.

We use the same hardware mechanism for both atomic counters and SSBO
atomics, so there's really no benefit to maintaining separate code to
handle each case.  Instead, we can just use Rob's shiny new NIR pass to
convert atomic_uints to SSBOs, and delete piles of code.

The ssbo_start section of the binding table becomes a combined ABO and
SSBO section, with ABOs first, then SSBOs.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/intel/compiler/brw_compiler.h

@@ -581,7 +581,6 @@ struct brw_stage_prog_data {
       uint32_t gather_texture_start;
       uint32_t ubo_start;
       uint32_t ssbo_start;
-      uint32_t abo_start;
       uint32_t image_start;
       uint32_t shader_time_start;
       uint32_t plane_start[3];