drm/i915/selftests: Verify mmap_gtt revocation on unbinding

Whenever, we unbind (or change fence registers) on an object, we must
revoke any and all mmap_gtt using the previous bindings. Those user PTEs
point at the GGTT which know points into a new object, the wrong object.
Ergo, those PTEs must be cleared so that any user access provokes a new
page fault.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107180601.30815-5-chris@chris-wilson.co.uk