drm/i915: Simplify ironlake reduced clock logic a bit

Check has_reduced_clock only once when setting dpll_hw_state, making the
code slightly more readable.

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/1458576016-30348-6-git-send-email-ander.conselvan.de.oliveira@intel.com