Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Address some WITNESS panics that occur when using the via driver.

Some of these cases should be safe in a non-atomic fashion, however
since all of the driver ioctls are locked, a lot of work is required to
fix it correctly. Just don't sleep now.

MFC after: 2 weeks

Import simple drm memory manager.

This is required for the VIA driver and at least some parts are needed
for GEM.

MFC after: 2 weeks