drm/i915/dp_mst: Use intel_connector vs. drm_connector pointer in intel_dp_mst.c

Follow the canonical way in intel_dp_mst.c, referencing a connector only
via a struct intel_connector pointer and naming this pointer 'connector'
instead of 'intel_connector', the only exception being the casting of
a drm_connector function parameter pointer to intel_connector, calling
the drm_connector pointer _connector.

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/20250108151916.491113-3-imre.deak@intel.com