drm/i915: fix lost FP_CB_TUNE setting for pch plls

commit de13a2e3f88a4da8e85063b6de37096795079e41
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Thu Sep 20 18:36:05 2012 -0300

    drm/i915: extract compute_dpll from ironlake_crtc_mode_set

missed the subtle adjustment of the FP1 register. Fix this up by
passing a pointer around instead of the value.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>