radv: Emit enqueued pipeline barriers on event write.

Since the CPU can read them we need to execute any GPU->CPU
flushes before the event is written.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108524


Fixes: f4e499ec "radv: add initial non-conformant radv vulkan driver"
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit d41c3cc0)