media: platform: video-mux: propagate format from sink to source

Propagate the v4l2_mbus_framefmt to the source pad when either a sink
pad is activated or when the format of the active sink pad changes.

Signed-off-by: Chris Lesiak <chris.lesiak@licor.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>