v4l2allocator: allow to import single dmabuf with multi planes
There is one case that v4l2src is connected to v4l2encoder. But three planes' buffer of camera belong to the same memory block. We want to set encoder output mode to DMABUF_IMPORT to avoid memory copy. Current gst_v4l2_allocator_import_dmabuf() does not support such case. So need to consider such case.