drm/i915/dp_mst: Fix ww mutex lock failure in for_each_joiner_candidate() loop

drm/i915/dp_mst: Fix ww mutex lock failure in for_each_joiner_candidate() loop

Handle modeset WW mutex lock failures due to contention properly.
Return -EDEADLK to the caller instead of attempting to lock the same
mutex again in a subsequent joiner candidate iteration, where the WW
mutex is already in a contended state.

This fixes the following warning:

WARNING: drivers/gpu/drm/drm_modeset_lock.c:298 at modeset_lock+0x1c0/0x210

triggered by ctx->contended.

Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Fixes: b506a8e18c972 ("drm/i915/dp_mst: Rework pipe joiner logic in compute_config")
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260218105708.845161-2-imre.deak@intel.com