drm/i915/dp: Cache max common lane count

drm/i915/dp: Cache max common lane count

Cache the maximum common lane count together with the common link
rates.

This is safe because the cached value is updated:
- during driver probe, before the connector is registered and can be
  used for mode validation or modesetting
- during resume, before output HW state readout can query it
- during connector detection, right after updating the sink/link
  capabilities

Caching the value allows detecting max common lane count changes in
a follow-up change and keeps the tracking of max common lane count
aligned with that of common rates.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260522160514.2628249-4-imre.deak@intel.com