drm/i915/mso: using joiner is not possible with eDP MSO

It's not possible to use the joiner at the same time with eDP MSO. When
a panel needs MSO, it's not optional, so MSO trumps joiner.

v3: Only change intel_dp_has_joiner(), leave debugfs alone (Ville)

Fixes: bc71194e

 ("drm/i915/edp: enable eDP MSO during link training")
Cc: <stable@vger.kernel.org> # v5.13+
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1668
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240614142311.589089-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
(cherry picked from commit 8b5a92ca

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>