- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

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