pan/bi: Special-case BLEND instruction emission for blend shaders

pan/bi: Special-case BLEND instruction emission for blend shaders

Blend shaders shouldn't use the blend descriptors stored in the FAU RAM
since this is what triggered the blend shader call in the first place.
The descriptor is instead extracted from the compiler inputs and passed
as a constant to the blend instruction.

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>