drm/i915/tgl: Keep FF dop clock enabled for A0

To ensure correct state data for compute workloads, we
need to keep the ff dop clock enabled.

References: HSDES#1606700617
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/20191015154449.10338-5-mika.kuoppala@linux.intel.com