chamelium: Get connector After plug to get EDID

[Why]
MST connectors can change connector IDs between plugs. Getting the EDID
prop requires a valid connector ID.

[How]
Get Connector AFTER a plug, just before it's being used.

Fixes: kms_chamelium@dp-edid-read

Signed-off-by: Mark Yacoub <markyacoub@chromium.org>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>