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: e45ba51ea4 "radv: add support for VK_EXT_conditional_rendering"
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 8c93ef5de9)

src/amd/vulkan/radv_cmd_buffer.c

@@ -4657,6 +4657,8 @@ void radv_CmdBeginConditionalRenderingEXT(
 		draw_visible = false;
 	}
 
+	si_emit_cache_flush(cmd_buffer);
+
 	/* Enable predication for this command buffer. */
 	si_emit_set_predication_state(cmd_buffer, draw_visible, va);
 	cmd_buffer->state.predicating = true;