v3d|vc4: Support offset query for multi-planar planes

Analogous to !26109 (merged) and !25603 (merged). Required to enable hardware plane offloading of NV12 and similar formats from video decoders.

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Draft as this is not enough to fix NV12 direct scanout on Mutter. There is an unrelated issue on my setup, but likely kernel related, see https://github.com/raspberrypi/linux/issues/5727