anv: Fix assert about source attrs.

Asserting slot >= 2 made sense when the URB read offset was always 1
(pair of slots).  Commit 566a0c43f0 made
it possible to read from the VUE header in slot 0, by adjusting the
offset to be 0.  So, this assert is now bogus.  Use the one from GL.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/intel/vulkan/genX_pipeline.c

@@ -379,8 +379,8 @@ emit_3dstate_sbe(struct anv_pipeline *pipeline)
          /* We have to subtract two slots to accout for the URB entry output
           * read offset in the VS and GS stages.
           */
-         assert(slot >= 2);
          const int source_attr = slot - 2 * urb_entry_read_offset;
+         assert(source_attr >= 0 && source_attr < 32);
          max_source_attr = MAX2(max_source_attr, source_attr);
          swiz.Attribute[input_index].SourceAttribute = source_attr;
       }