KMS property fetch complains that 'DPMS' is an enum

Starting Weston with the DRM backend will complain for every connector:

[18:04:23.299] DRM: expected property DPMS to not be an enum, but it is; ignoring

This should be fairly trivially fixed by recognising the enum values, which are On, Standby, Suspend, and Off. We would only use on and off.