[AGPGART] Fix serverworks TLB flush.

[AGPGART] Fix serverworks TLB flush.
Go back to what 2.4 kernels used to do here, as if this hits,
the kernel just hangs indefinitly.

Actually an improvement over 2.4 - we now break; out of the loop
instead of just printing messages on timeouts.

Signed-off-by: Dave Jones <davej@redhat.com>