Don't use planes currently in-use on other CRTCs on first commit

When doing the first commit, libliftoff will try to disable all candidate planes. This is necessary to make sure we start atomic test commits from a clean state, and are able to incrementally come up with a combination which works.

However, disabling a plane currently in-use on another CRTC (e.g. a primary plane) will result in failed commits. We can't steal that plane without removing it from the other CRTC first.

Add a new liftoff_plane.crtc_id field populated with the current CRTC ID for the plane.

No test introduced here sadly because we don't yet have the infrastructure needed to simulate multiple CRTCs.