backend/drm: Add support for hotplug_mode_update connector property
This connector property is used by virtual graphics drivers to indicate that a virtual display has changed its list of supported resolutions and updated its preferred mode. In the event of a hotplug the connector is considered to be "re-plugged" without disconnecting first. This allows virtual displays to be resized automatically when their window size changes.
Signed-off-by: Ian Forbes ian.forbes@broadcom.com