radv: Do a cache flush if needed before reading predicates.
This caused random failures for two conditional rendering tests:
dEQP-VK.conditional_rendering.draw_clear.draw.update_with_rendering_discard
dEQP-VK.conditional_rendering.draw_clear.draw.update_with_rendering_no_discard
These wrote the predicate with the vertex shader, did a barrier and then started the conditional rendering. However the cache flushes for the barrier only happen on first draw, so after the predicate has been read.
Fixes: e45ba51e "radv: add support for VK_EXT_conditional_rendering"
Edited by Bas Nieuwenhuizen