drm: Introduce "Virtual" connectors and encoders

This will allow us to attach various properties specific to virtual
monitors in the future.

Note that we don't export an EDID property for "Virtual" connectors.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>