Avoid dangerous race handling of destroyed vma by unbinding all vma
instead. Unfortunately, this stops us from trying to be clever and only
doing the minimal change required, so on first use of scanout we may
encounter an annoying stall as it transitions to a new cache level.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112413Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202174310.2630302-1-chris@chris-wilson.co.uk