drm/i915/dp: Restore the missing check for intel_dp_has_joiner

drm/i915/dp: Restore the missing check for intel_dp_has_joiner

Commit ad121a62d566 ("drm/i915/dp: Rework pipe joiner logic in mode_valid")
replaced intel_dp_num_joined_pipes() with an explicit joiner candidate
iteration. The previous code implicitly checked for DP joiner capability
via intel_dp_has_joiner(), but this check was lost during the refactor.

Restore the missing intel_dp_has_joiner() check in intel_dp_can_join() so
that DP specific joiner conditions are taken into account.

v2: Derive intel_dp from intel_attached_dp(). (Imre)

Fixes: ad121a62d566 ("drm/i915/dp: Rework pipe joiner logic in mode_valid")
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260206044753.808631-1-ankit.k.nautiyal@intel.com