Fix enabling MST displays in the future

A quick note here: the only reason this isn't currently broken in the X server is because there aren't many situations in the kernel right now where we have CRTCs reserve resources that other CRTCs will need in order to be enabled. Once https://patchwork.freedesktop.org/series/51414/ goes through, this will break enabling MST displays in X since the current laziness of modesetting not disabling CRTCs will cause us to hit issues where we can't enable new CRTCs due to old inactive CRTCs in the atomic VCPI state still being enabled (seen on my current WIP branches with this patch series). This is definitely going to need to go into a bugfix release.