panfrost: Move the blend constant mask extraction out of make_fixed_blend_mode()

This way we can get a constant mask for the blend shader case too.

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/7066>
3 files changed