drm/i915: split out PCH refclk update code

We ought to be calling this from our DPMS routines as well as global
state may change and we need to enable/disable clocks.  So split out the
code in preparation for further changes.

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