anv: ignore pColorBlendState if all color attachments of the subpass are unused

anv: ignore pColorBlendState if all color attachments of the subpass are unused

According to Vulkan spec:

  "pColorBlendState is a pointer to an instance of the
   VkPipelineColorBlendStateCreateInfo structure, and is ignored if the
   pipeline has rasterization disabled or if the subpass of the render pass the
   pipeline is created against does not use any color attachments."

Fixes tests from CL#2505:

   dEQP-VK.renderpass.*.simple.color_unused_omit_blend_state

v2:
- Check that blend is not NULL before usage.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>