drm/i915: Fold intel_ironlake_limit() into clock computation function

The function intel_ironlake_limit() is only called by the crtc compute
clock path. By merging it into ironlake_compute_clocks(), the code gets
clearer, since there's no more if-ladders to follow.

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/1458576016-30348-4-git-send-email-ander.conselvan.de.oliveira@intel.com