[media] videobuf2-core: call __setup_offsets only for mmap memory type

__setup_offsets fills the v4l2_planes' mem_offset fields, which is only valid
for V4L2_MEMORY_MMAP type buffers. For V4L2_MEMORY_DMABUF and _USERPTR buffers,
this incorrectly overwrites the fd and userptr fields.

Reported-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Pawel Osciak <pawel@osciak.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>