drm/i915: Extract intel_dp_need_bigjoiner()

Suck the "do we need bigjoiner?" checks into a helper instead of
duplicating them in two differentt places.

v2: s/use/need/ (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210913144440.23008-4-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>