media: video-mux: Read CSI2 config from FW, and pass to receiver

There is no obligation for all source devices on a video-mux to
require the same bus configuration, so read the configuration
from the sink ports, and relay via get_mbus_config on the source
port.
If the sources support get_mbus_config, then call that first.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>