drm/i915: split PCH clock gating init

Ibex Peak and CougarPoint already require a different setting (added
here), and future chips will likely follow that precedent.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>