drm/i915/tgl: Add extra hdc flush workaround

In order to ensure constant caches are invalidated
properly with a0, we need extra hdc flush after invalidation.

v2: use IS_TGL_REVID (Chris)

References: HSDES#1604544889
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015154449.10338-4-mika.kuoppala@linux.intel.com