WIP: rust: kms/crtc: Add DriverCrtcEphemeralData

There's another class of state that many drivers keep track of, usually for
intermediate state that needs to be tracked during the middle of a commit.
There isn't really a name for this kind of state, so I've named it
EphemeralData. This data is not expected to be accessible outside of a
modeset context, and should only be used for modesetting state that changes
within the process of a commit rather than from commit to commit.

Signed-off-by: Lyude Paul <lyude@redhat.com>