[gem] Manage the ringbuffer from the kernel in the GEM case.

This requires that the X Server use the execbuf interface for buffer
submission, as it no longer has direct access to the ring.  This is
therefore a flag day for the gem interface.

This also adds enter/leavevt ioctls for use by the X Server.  These would
get stubbed out in a modesetting implementation, but are required while
in an environment where the device's state is only managed by the DRM while
X has the VT.