anv: Fix assert about source attrs.

anv: Fix assert about source attrs.

Asserting slot >= 2 made sense when the URB read offset was always 1
(pair of slots).  Commit 566a0c43f0b9fbf5106161471dd5061c7275f761 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>