drm/radeon/kms: fix r600/r700 cs checker to avoid double kfree

radeon_cs kfree the tracker structure but for r6xx/r7xx we want
to kfree it inside the parse function because we share it with
the UMS code path. Set tracker to NULL after freeing it will
avoid double free.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>