compositor-drm: Remove legacy plane and vblank usage

As of the previous commit, we never create state which uses overlay
planes on non-atomic drivers. We can thus remove the calls to
drmModeSetPlane.

The only time we ever waited for vblank events was when we had called
drmModeSetPlane and needed to make sure we waited until it was active.
We can thus also remove all the vblank event machinery.

Signed-off-by: Daniel Stone <daniels@collabora.com>