gpu: ipu-v3: ipu-dmfc: Protect function ipu_dmfc_init_channel() with mutex

To avoid race condition issue, we should protect the function
ipu_dmfc_init_channel() with the mutex dmfc->priv->mutex, since it
configures the register DMFC_GENERAL1 at runtime which contains
several control bits for various display channels.  This matches
better with fine grained locking logic in upper layer.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>