drm/i915/intel_dp: Add support for forcing ultrajoiner

Allow forcing ultrajoiner through debugfs.

v2: Minor refactoring of switch case logic. (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930163549.416410-14-ankit.k.nautiyal@intel.com