media: vimc: stream: fix style of argument description

As in "Function parameters" at doc-guide/kernel-doc.rst, "the
continuation of the description should start at the same column as the
previous line". Make the @producer_pixfmt comply with that.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>