gpu: ipu-csi: Check for field type alternate

When the CSI is receiving from a bt.656 bus, include a check for
field type 'alternate' when determining whether to set CSI clock
mode to CCIR656_INTERLACED or CCIR656_PROGRESSIVE.

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