media: imx: capture: Store v4l2_pix_format in imx_media_video_dev

The imx_media_video_dev structure stores the active format on the video
node in an instance of v4l2_format. This wastes memory when all we need
is the information contained in the smaller v4l2_pix_format
sub-structure. Replace v4l2_format by v4l2_pix_format to save memory.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>