drm/imx: don't skip DP channel disable for background plane

[ Upstream commit 7bcde275eb1d0ac8793c77c7e666a886eb16633d ]

In order to make sure that the plane color space gets reset correctly.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>