drm/i915: mark display port DPMS state as 'ON' when enabling output

The display port DPMS state is tracked internally in the display port
driver so that when a hotplug event comes along, the driver can know
whether to try retraining the link. This doesn't work well if the
driver never sets the DPMS state to ON when the output is enabled.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>