radeon: Don't mess up page flipping when a file descriptor is closed.

There can still be other contexts that may use page flipping later on, so do
just unilaterally 'clean it up', which could lead to the wrong page being
displayed, e.g. when running 3D apps with a GLX compositing manager such as
compiz using page flipping.

Signed-off-by: Dave Airlie <airlied@linux.ie>