drm/i915: Extract intel_bigjoiner_adjust_timings()

Deduplicate the code to convert the full timings to
per-pipe timings for bigjoiner usage.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223131315.18016-7-ville.syrjala@linux.intel.com