i965: Move the post-HiZ-clear flush/stall to intel_hiz_exec

i965: Move the post-HiZ-clear flush/stall to intel_hiz_exec

This also changes it to be predicated so we only do the flush/stall on
clears and HiZ resolves.  The docs only say it's needed for clears but
empirical evidence says it's also needed for HiZ resolves.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>