drm/i915: split aux_clock_divider logic in a separated function for reuse.

Prep patch for reuse aux_clock_divider with EDP_PSR_AUX_CTL setup.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>