i965/gen6/blorp: Remove redundant HiZ workaround

i965/gen6/blorp: Remove redundant HiZ workaround

Commit 1a92881 added extra flushes to fix a HiZ hang in
WebGL Google Maps. With the extra flushes emitted by the previous two
patches, the flushes added by 1a92881 are redundant.

Tested with the same criteria as in 1a92881: by zooming in and out
continuously for 2 hours on Sandybridge Chrome OS (codename
Stumpy) without a hang.

CC: Kenneth Graunke <kenneth@whitecape.org>
CC: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>