drm/dp/mst: fix a possible memory leak in fetch_monitor_name()

drm_dp_mst_get_edid call kmemdup to create mst_edid. So mst_edid need to be
freed after use.

Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220516032042.13166-1-hbh25y@gmail.com