panvk: Initialize the blend shader logic

panvk: Initialize the blend shader logic

The blitter logic rely on blend shaders when formats are not blendable.
We need to initialize the blend shader logic before we can use the
blitter.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12095>