nv50: reset TFB bufctx when we no longer hold a reference to the buffers

nv50: reset TFB bufctx when we no longer hold a reference to the buffers

This fix is analogous to commit ff085d014.

This fixes some use-after-free situations in dEQP when an xfb state is
removed, and then a clear is triggered, which only does a partial
validation. It would attempt to read the no-longer-valid buffers,
resulting in crashes.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>