media: atomisp: properly implement g_fmt

The current logic only initializes pipe->pix after setting up a
pipeline - e. g. after start streaming.

While it makes sense to get the format of the pipeline, when
it is set, this breaks support for generic applications, as they
rely on getting the current sensor format (which is usually the
highest resolution format).

So, implement a call to the sensor's get_fmt, when this is called
before setting up a pipeline.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>