[media] v4l: vsp1: Use __vsp1_video_try_format to initialize format at init time

Reuse the runtime logic to initialize the default format instead of
open-coding it. This ensures coherency between intialization and
runtime.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>