gpu: ipu-v3: Add __ipu_idmac_reset_current_buffer()

Adds __ipu_idmac_reset_current_buffer() that resets a channel's
internal current buffer pointer so that transfers start from buffer
0 on the next channel enable.

This operation is required for channel linking to work correctly,
for instance video capture pipelines that carry out image rotations
will fail after the first streaming unless this function is called
for each channel before re-enabling the channels.

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>