mesa: only verify that enabled arrays have backing buffers

mesa: only verify that enabled arrays have backing buffers

We were previously also verifying that no backing buffers were available
when an array wasn't enabled. This is has no basis in the spec, and it
causes GLupeN64 to fail as a result.

Fixes: c2e146f487 ("mesa: error out in indirect draw when vertex bindings mismatch")
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
(cherry picked from commit 7c16552f8dcc869b14cf7ef443a1b5de83b07973)