V4L/DVB (12525): soc-camera: prohibit geometry change with initialised buffers

Prohibit S_FMT and S_CROP with a different window width or height after video
buffer initialisation. This simplifies the work to be done in specific host and
client drivers, and it doesn't seem to make much sense to allow these changes.
We do however allow S_CROP with equal width and height to just move the window,
this doesn't affect video buffer management and is usually easy enough to
implement.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>