drm/i915/dp_mst: Simplify using to_intel_display() passing it an intel_connector pointer

Simplify the use of to_intel_display() in intel_dp_mst.c passing it the
already available intel_connector pointer, instead of looking up a
drm_device pointer for the same purpose.

Suggested-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250108151916.491113-4-imre.deak@intel.com