drm/panfrost: Fix shrinker list corruption by madvise IOCTL

Calling madvise IOCTL twice on BO causes memory shrinker list corruption
and crashes kernel because BO is already on the list and it's added to
the list again, while BO should be removed from from the list before it's
re-added. Fix it.

Cc: stable@vger.kernel.org
Fixes: 013b6510 ("drm/panfrost: Add madvise and shrinker support")
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>