drm/i915: Merge ironlake_get_refclk() into its only caller

A previous patch made ironlake_get_refclk() very simple, so merge
it into its only caller.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458576016-30348-3-git-send-email-ander.conselvan.de.oliveira@intel.com