radv: Do a cache flush if needed before reading predicates.

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: e45ba51ea45 "radv: add support for VK_EXT_conditional_rendering"
Reviewed-by: Dave Airlie <airlied@redhat.com>