panfrost: Fix AFBC on Bifrost v6

The AFBC layout of RT/ZS-extension descriptors on Bifrost v6 matches the
v7 one except for the Block Format field. Update the set_buf() functions
accordingly.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <!8328>
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