drm/i915: Clean up .get_aux_clock_divider() functions

drm/i915: Clean up .get_aux_clock_divider() functions

Now that the mess with AUX clock divder rounding is sorted out and
we have both cdclk and rawclk cached in dev_priv, we can clean up
the .get_aux_clock_divider() functions a bit.

The main thing here is just calling ilk_get_aux_clock_divider()
from hsw_get_aux_clock_divider() except for the LPT:H special
case.

We could go further and call g4x_get_aux_clock_divider() from
ilk_get_aux_clock_divider() for the PCH ports, but I'm sure Jani
would object, so leave that be.

While at it repeat the comment where the AUX clock comes from
in ilk_get_aux_clock_divider().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456932138-14004-6-git-send-email-ville.syrjala@linux.intel.com