media: imx: utils: Make imx_media_pixfmt handle variable number of codes

The imx_media_pixfmt structures include a codes member that stores
media bus codes as a fixed array of 4 integers. The functions dealing
with the imx_media_pixfmt structures assume that the array of codes is
terminated by a 0 element. This mechanism is fragile, as demonstrated
by several instances of the structure containing 4 non-zero codes.

Fix this by turning the array into a pointer, and providing an
IMX_BUS_FMTS macro to initialize the codes member with a guaranteed 0
element at the end.

[Fixed a NULL deref of the codes pointer in a couple places]
[Added more comments for the struct imx_media_pixfmt members,
 including a bold NOTE! for future developers that codes pointer
 is NULL for the in-memory-only formats]

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>