Enumerate and expose overlay planes from QKmsDevice (kmsconvenience)

Due to the split made in 5.9 (QKmsDevice in kmsconvenience, shared between linuxfb and the relevant eglfs backends), all connector discovery and similar functionality is conveniently collected in one place.

This can and should be enhanced to include planes as well. Like with screens, QKmsDevice will not know anything how (or if) those will be used in the subclasses.

Independent of atomic since this only uses resource enumeration.