gpu: ipu-v3: Add ipu_idmac_get_current_buffer function

This function returns the currently active buffer (0 or 1)
of a double buffered IDMAC channel. It is to be used by the
CSI driver.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

diff --cc drivers/gpu/ipu-v3/ipu-common.c
Simple merge

diff --cc include/video/imx-ipu-v3.h
Simple merge