drm/i915: split refclk code out of ironlake_crtc_mode_set

Just a cleanup to make the mode_set function more manageable.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-By: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-By: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>