anv: do not get rid of empty/useless fragment shaders

This was an optimization done a while ago that doesn't seem to be having much of an impact anymore, and on the other hand, causes all sorts of breakage with queries, as many of our HW counters don't get incremented when rasterization is disabled.

This fixes a bunch of issues Zink has with ANV, but more importantly, it fixes upcoming CTS tests: dEQP-VK.transform_feedback.primitives_generated_query..empty_frag. dEQP-VK.transform_feedback.primitives_generated_query..no_attachment. dEQP-VK.transform_feedback.primitives_generated_query..color_write_disable_