va: allocator: Use derived images only if not mapped for reading.

va: allocator: Use derived images only if not mapped for reading.

Derived images are direct maps to surfaces bits, but in Intel Gen7 to
Gen9, that memory is not cachable, thus reading can be very slow (it
might produce timeout is tests such as fluster).

This patch tries first to define if derived images are possible, and
later use them only if mapping is not for reading.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2128>