weston: add force-on option for DRM

Add a new boolean output section key "force-on". When set to true, the
output will be enabled regardless of connector status. This is the
opposite of the mode=off setting.

Forcing connectors on is useful in special circumstances: avoid output
configuration changes due to hotplug e.g. with KVM switches, or hardware
with unreliable connector status readout for example.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ian Ray <ian.ray@ge.com>