drm/i915/display_debugfs: Allow force joiner only if supported

Currently we support joiner only for DP encoder.
Do not create the debugfs for joiner if DP does not support the joiner.
This will also help avoiding cases where config has eDP MSO, with which
we do not support joiner.

v2: Check for intel_dp_has_joiner and avoid creating debugfs if not
supported. (Ville)
v3: Remove HAS_BIGJOINER check. (Ville)
v4: Reverse checks for connector type and intel_dp_has_joiner(). (Ville)
v5: Drop the local variable intel_dp and use intel_attached_dp() directly.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240926134322.3728021-3-ankit.k.nautiyal@intel.com