pan/bi: Model writemasks correctly

pan/bi: Model writemasks correctly

We don't handle partial write masks in the backend yet, so for now we
can't pretend we do, else we'll have RA bugs. Fixes

dEQP-GLES2.functional.fragment_ops.blend.rgb_func_alpha_func.src.constant_color_dst_alpha

Fixes: b2c6cf2b6db1 ("pan/bi: Eliminate writemasks in the IR")
Cc: 20.3 <mesa-stable>
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reported-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7527>