drm/i915: extract compute_dpll from ironlake_crtc_mode_set

Too many lines just to compute the value of a single variable, so
move this to its own function.

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