agx: Defeature indirect vbufs

vb_mask can include garbage vbufs, we can't rely on it. This will
prevent a regression when switching to u_blitter based clears. This is
also simpler and shrinks the VS shader key so all in all a good thing.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18380>

src/asahi/compiler/agx_compiler.h

@@ -696,6 +696,9 @@ agx_index
 agx_indexed_sysval(agx_context *ctx, enum agx_push_type type, enum agx_size size,
       unsigned index, unsigned length);
 
+agx_index
+agx_vbo_base(agx_context *ctx, unsigned vbo);
+
 /* Routines defined for AIR */
 
 void agx_print_instr(agx_instr *I, FILE *fp);