When we're predicting state, i.e. when having posted an update while
avoiding reading KMS state, copy the predicted state, update the actual
state, and check that the predicted state matches the newly updated one.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2159>
This contains a copy of a drmModeModeInfo, describing a mode. It also
has an unused pointer to the impl device it is associated with. It'll
later be used to get a blob ID for the mode.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1488>