intel: add intel_render_pass_{attachment,subpass}

Make intel_render_pass own an array of intel_render_pass_attachment and an
array of intel_render_pass_subpass.  Every subpass references the attachment
array for its outputs.

It gets a bit more clear that intel_fb is for intel_render_pass just like
intel_desc_set is for intel_desc_layout.  It has always been so at the API
level, but it was harder to see.

This is in preparation for an upcoming header change.
diff --git a/icd/intel/cmd.h b/icd/intel/cmd.h
index 720e6a2..9d9a70b 100644
--- a/icd/intel/cmd.h
+++ b/icd/intel/cmd.h
@@ -188,6 +188,7 @@
 
     bool render_pass_changed;
     const struct intel_render_pass *render_pass;
+    const struct intel_render_pass_subpass *render_pass_subpass;
     const struct intel_fb *fb;
     VkRenderPassContents render_pass_contents;