zink: Always set a blend state for shader-db

zink: Always set a blend state for shader-db

If we're compiling shaders in shader-db, with shader-db's ./run and
ZINK_DEBUG=shaderdb, we won't get much state set on the graphics pipeline, since
shader-db doesn't actually do any rendering. For a driver like RADV, that is
*almost* ok... Since we use dynamic vertex input, we don't need to make up any
state for vertex inputs; since we use dynamic rendering, we don't need to make
up any render attachments. All of that being said, we *do* need to make up a
blend state to ensure that the Vulkan driver doesn't optimize away all of
store_derefs in the fragment shader (and in turn, optimize the entire fragment
shader away, if there are no image/SSBO writes.) So set the obvious blend state,
fixing fragment shaders in shader-db with zink + radv.

I don't know why other people would want to use Zink with shader-db, but for me
it's an easy way to test ACO, at least until radeonsi gains aco support.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22948>