vk: Set cb_state to NULL at cmd buffer create time

Dynamic color/blend state can be NULL in case we're not rendering to
color targets (only output to depth and/or stencil). Initialize
cmd_buffer->cb_state to NULL so we can reliably detect whether it's been
set or not.

src/vulkan/device.c

@@ -2297,6 +2297,7 @@ VkResult anv_CreateCommandBuffer(
    cmd_buffer->device = device;
    cmd_buffer->rs_state = NULL;
    cmd_buffer->vp_state = NULL;
+   cmd_buffer->cb_state = NULL;
    memset(&cmd_buffer->descriptors, 0, sizeof(cmd_buffer->descriptors));
 
    result = anv_batch_bo_create(device, &cmd_buffer->last_batch_bo);