Revert "drm/i915/tgl: Add extra hdc flush workaround"

This reverts commit 36a6b5d9.

The commit takes care Wa_1604544889 which was fixed on a0 stepping based on
a0 replan. So no SW workaround is required on any stepping now.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Caz Yokoyama <caz.yokoyama@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Fixes: 36a6b5d9 ("drm/i915/tgl: Add extra hdc flush workaround")
Link: https://patchwork.freedesktop.org/patch/msgid/1c751032ce79c80c5485cae315f1a9904ce07cac.1583359940.git.caz.yokoyama@intel.com