softpipe: Implement ATOMFADD and enable cap TGSI_ATOMFADD

Gert Wollny requested to merge gerddie/mesa:softpipe-enable-tgsi-atomadd into master

This enables the following piglits with PASS: nv_shader_atomic_float/execution/ shared-atomicadd-float shared-atomicexchange-float ssbo-atomicadd-float ssbo-atomicexchange-float

Merge request reports