anv/pipeline: Don't look at blend state unless we have an attachment

Without this, we may end up dereferencing blend before we check for
binding->index != UINT32_MAX.  However, Vulkan allows the blend state to
be NULL so long as you don't have any color attachments.  This fixes a
segfault when running The Talos Principal.

Fixes: 12f4e00b
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Alex Smith <asmith@feralinteractive.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
(cherry picked from commit c8949e24)