st/mesa: Add NV12 lowering to PIPE_FORMAT_R8_G8B8_420_UNORM

Some GPUs can sample biplanar formats like NV12 natively, returning
the YUV values. Add a lowering type that uses that for sampling and
relies on existing colorspace conversions.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6693>
9 files changed