media: rp1: csi2: Use get_frame_desc to get CSI-2 VC and DT

Use get_frame_desc pad op for asking the CSI-2 VC and DT from the source
device driver, instead of hardcoding to VC 0, and getting the DT from a
formats table. To keep backward compatibility with sources that do not
implement get_frame_desc, implement a fallback mechanism that always
uses VC 0, and gets the DT from the formats table, based on the CSI2's
sink pad's format.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>