drm/i915: Move fp divisor calculation into ironlake_compute_dpll()

Follow what is done in i8xx_compute_dpll() and i9xx_compute_dpll() and
move the lower level details of setting crtc_state->dpll_hw_state into
the _compute_dpll() function.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458576016-30348-11-git-send-email-ander.conselvan.de.oliveira@intel.com