drm/dp/mst: move GUID storage from mgr, port to only mst branch

drm/dp/mst: move GUID storage from mgr, port to only mst branch

BugLink: http://bugs.launchpad.net/bugs/1553179
commit 5e93b8208d3c419b515fb75e2601931c027e12ab upstream.

Previous implementation does not handle case below: boot up one MST branch
to DP connector of ASIC. After boot up, hot plug 2nd MST branch to DP output
of 1st MST, GUID is not created for 2nd MST branch. When downstream port of
2nd MST branch send upstream request, it fails because 2nd MST branch GUID
is not available.

New Implementation: only create GUID for MST branch and save it within Branch.

Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>