drm/i915: save/restore GMBUS freq across suspend/resume on gen4

Should probably just init this in the GMbus code all the time, based on
the cdclk and HPLL like we do on newer platforms.  Ville has code for
that in a rework branch, but until then we can fix this bug fairly
easily.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76301
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Nikolay <mar.kolya@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>