pan/bi: Rework blend descriptor access handling

pan/bi: Rework blend descriptor access handling

The current logic assumes blend descriptors are always retrieved from
the blend descriptor slots present in the FAU RAM, but this assumption
no longer stands when we add blend shaders to the mix. In that case we
need to use an 'opaque blend' whose descriptor is passed through
embedded constants.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7151>