drm/i915: Extract intel_splitter_adjust_timings()

Let's not replicate the same piece of code to expand
the MSO segment timings to full width in many places.
Pull it into a helper

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223131315.18016-6-ville.syrjala@linux.intel.com