drm/i915: Simplify ilk_get_aux_clock_divider

Now that we are "extracting" the cdclk frequency on ILK-IVB we
can also simplify ilk_get_aux_clock_divider() to calculate the
divider based on cdclk instead of hardcoding the values.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>