gpu: drm: drm_dp_mst_topology.c: Fix improper use of strncat

Fixed wrong usage of strncat, switched to strlcpy.
While sending the string size to function to reduce
the potential for misuse in future.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>