drm/dp_mst: Rename drm_dp_add_port and drm_dp_update_port

The names for these functions are rather confusing. drm_dp_add_port()
sounds like a function that would simply create a port and add it to a
topology, and do nothing more. Similarly, drm_dp_update_port() would be
assumed to be the function that should be used to update port
information after initial creation.

While those assumptions are currently correct in how these functions are
used, a quick glance at drm_dp_add_port() reveals that drm_dp_add_port()
can also update the information on a port, and seems explicitly designed
to do so. This can be explained pretty simply by the fact that there's
more situations that would involve updating the port information based
on a link address response as opposed to a connection status
notification than the driver's initial topology probe. Case in point:
reprobing link addresses after suspend/resume.

Since we're about to start using drm_dp_add_port() differently for
suspend/resume reprobing, let's rename both functions to clarify what
they actually do.

Cc: Juston Li <juston.li@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Harry Wentland <hwentlan@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20190903204645.25487-18-lyude@redhat.com