tests: modetest: Accept connector names in addition to connector IDs

Allow connector names to be used in the specification of the -s option.
This requires storing the string passed on the command-line so that it
can later be resolved to a connector ID (after the DRM device has been
opened).

Connector names are constructed from the connector type name and
connector type ID using the same format as used internally in the
Linux kernel.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>