drm/sun4i: Fix format mask in DE2 driver

Format mask is one bit too short. Fix it.

Fixes: 9d75b8c0 (drm/sun4i: add support for Allwinner DE2 mixers)

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201060550.10392-2-jernej.skrabec@siol.net