Manage fences in user-mode bufmgr_fake to clean buffers

When using bufmgr_fake without DRM, the X server idles the ring whenever it
wants to wait for something to complete (brutal, but effective). In this
case, bufmgr_fake must treat the pending fence as having passed. However, it
wasn't recording the fences as it emitted them, nor cleaning buffers as they
passed.

Signed-off-by: Keith Packard <keithp@keithp.com>