drm/i915: Fix an overlay regression from 7e7d76c3

When separating out the prepare/commit into its own separate functions
we overlooked that the intel_crtc->dpms_mode was being used elsewhere to
check on the actual status of the pipe.

Track that bit of logic separately from the actual dpms mode, so there
is no confusion should we be able to handle multiple dpms modes, nor
any semantic conflict between prepare/commit and dpms.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>