drm/i915: Merge ironlake_compute_clocks() and ironlake_crtc_compute_clock()

Merge ironlake_compute_clocks() into ironlake_crtc_compute_clock() so
the clock computation logic is all in one place. The resulting function
is still quite simple. Follow up patches will make the similar code for
GMCH platforms look similar.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458576016-30348-12-git-send-email-ander.conselvan.de.oliveira@intel.com