frontend/dri: Implement mapping individual planes.

frontend/dri: Implement mapping individual planes.

It is kinda surprising that

image2 = fromPlanar(image, 2, NULL)
mapImage(..., image2, ...)

does not map the third plane.

This implements that behavior in the case where the DRI frontend
lowers the multi-planar textures.

In the case it doesn't this would need driver support. AFAIU at
least etnaviv is impacted, and while it looks possible, I don't
have the etnaviv knowledge to implement it.

Instead of silently returning weird results (either always plane 0
or possibly something interleaved) this adds an error return on
mapping multi-planar textures otherwise.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5200>