drm/i915/tgl: Wa_1409170338

Avoid possible hang in tsg,vfe units by keeping
l3 clocks runnings.

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/20191015154449.10338-7-mika.kuoppala@linux.intel.com