kms: drmModeGetFoo calls should have NULL return checks
Almost all drmModeGetFoo calls return a pointer to a struct with the requested data, or NULL is one of the underlying ioctls fail.
Mutter should be prepared to handle these calls returning NULL and not assume that they will always succeed as it currently does in various places.
Note !713 (merged) fixes a few of these which were hit on GPU unplug, but we need to check all drmModeGet* calls and add error handling to all of them.