drm/i915: Turn get_aux_clock_divider() into per-platform vfuncs

A tiny clean-up to allow better code separation between platforms.

v2: Fix comment placement (put in in i9xx_get_aux_clock_divider()) and
    nuke the outdated PCH eDP comment (Jani Nikula)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>