drm/i915: Use g4x_get_aux_clock_divider() for VLV/CHV

With the hrawclk frequency cached in dev_priv, we can simply use
g4x_get_aux_clock_divider() for VLV/CHV.

v2: Rebase due to IS_VALLYVIEW vs. IS_CHERRYVIEW split

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/1456932138-14004-4-git-send-email-ville.syrjala@linux.intel.com