drm/mst: cached EDID for logical ports (v2)

Logical ports are never going to have EDID changes,
they are used for the internal ports on MST monitors.

We cache the EDIDs from these to save time at MST probe.

v2: drop misplace tile property line, meant for other patch.

Signed-off-by: Dave Airlie <airlied@redhat.com>