media: videobuf2-v4l2: move __fill_v4l2_buffer() function

Move the __fill_v4l2_buffer() to before the vb2_queue_or_prepare_buf()
function to prepare for the next two patches.

No other changes.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>