drm/i915: refactor cpu eDP PLL handling a bit

This prepares a bit for the next big patch, where we switch the
semantics of the different clocks in the pipe config around.

Since I've broken cpu eDP PLL handling in the first version I've
figured some refactoring is in order.

Split out on request from Paulo Zanoni.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>