drm/i915: Track the psr dp connector in dev_priv->psr.enabled

Trying to fish that one out through looping is a bit a locking
nightmare. So just set it and use it in the work struct.

v2:
- Don't Oops in psr_work, spotted by Rodrigo.
- Fix compile warning.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>