drm/sun4i: Fix the high buffer address mask

The highest 3bits of the 4 layers buffers are all part of the same
register. However, our mask computation was wrong, leading to all the
lowest register bits being removed when we use regmap_update_bits, which
will lead to the buffers being set to some random part of the RAM.

Fix our mask.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>