Blend shaders can be shared among blend states, so let's move the blend
shader one level up so we don't have to re-create/re-compile shaders
when another blend state already asked for it.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <mesa/mesa!7066>