A client may map dmabufs without the intention to either read or write
to the memory. One example is clients wanting to use the
`gst_video_frame_map()` helper function.
Thus, in order to make buffers from `GstVaDmabufAllocator` conveniently
usable, ignore the modifier check if the client specified neither
`GST_MAP_READ` nor `GST_MAP_WRITE`.
Also skip the `va_sync_surface()` call in that case, as it's likely only
needed for CPU reads/writes.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5965>
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