anv/blorp: Do more flushing around HiZ clears

We make the flush after a HiZ clear unconditional and add a flush/stall
before the clear as well.

Cc: mesa-stable@lists.freedesktop.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107760


Reviewed-by: Chad Versace <chadversary@chromium.org>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
(cherry picked from commit 62378c5e)