drm/i915: Handle cdclk limits on broadwell.

As the comment indicates this can only fail gracefully when
called from compute_config. Fortunately this is now what's happening,
so the fixme can be removed and the DRM_ERROR downgraded.

Link: http://patchwork.freedesktop.org/patch/msgid/1448360945-5723-3-git-send-email-maarten.lankhorst@linux.intel.com


Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>