drm/i915: Fix GMBUSFREQ on vlv/chv

vlv_cdclk_freq is in kHz but we need MHz for the GMBUSFREQ divider.

This is a regression from:
commit f8bf63fd
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Fri Jun 13 13:37:54 2014 +0300

    drm/i915: Kill duplicated cdclk readout code from i2c

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>