Handling DRM master transitions cooperatively

[Simon Ser]

FWIW, I've just hit a case where a compositor leaves a "rotation" KMS
prop set behind, then Xorg tries to startup and fails because it doesn't
reset this prop. So none of this is theoretical.

I still think a "reset all KMS props to an arbitrary default value" flag
in drmModeAtomicCommit is the best way forward. I'm not sure a user-space
protocol would help too much.