drm/i915/tgl: Re-enable rc6

We think that we got rc6 problems sorted out. Flip the switch
and let CI expose our tendency to naive optimism.

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/20190913200638.31939-1-chris@chris-wilson.co.uk