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    nuke the outdated PCH eDP comment (Jani Nikula)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: NDamien Lespiau <damien.lespiau@intel.com>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>