panfrost/va: implement fquantize2f16 ftz on valhall
Bifrost implements this with a ftz pseudo-modifier in the conversion instructions that is lowered to a ftz flag in the clause header. Valhall doesn't have clauses, and needs a separate flush instruction with a ftz modifier.
Fixes dEQP-VK.spirv_assembly.instruction.compute.opquantize.*