drm/dp_mst: Fix some formatting in drm_dp_add_port()

drm/dp_mst: Fix some formatting in drm_dp_add_port()

Reindent some stuff, and split some stuff across multiple lines so we
aren't going over the text width limit.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@redhat.com>
Cc: Jerry Zuo <Jerry.Zuo@amd.com>
Cc: Juston Li <juston.li@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111005343.17443-2-lyude@redhat.com