compositor-drm: Define DPMS property as an enum

The DPMS connector property is an enum property in KMS, which made our
property handling complain at startup as we weren't defining its enums.
Fix our definition so we parse the enum values.

The only user of the property is the legacy path, which can continue
using fixed values as those values are part of the KMS ABI. The atomic
path does not need any changes, since atomic uses routing and CRTC
active to determine the connector's power state, rather than a property.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Fixes: https://gitlab.freedesktop.org/wayland/weston/issues/125


Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>