intel_agp: Don't oops with zero stolen memory

When "onboard video memory" is set do "disabled" in BIOS on Asus P4P800-VM
board (i865G), kernel oopses with memory corruption:
https://bugs.freedesktop.org/show_bug.cgi?id=28430



Fix that by cleanly aborting the initialization.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Eric Anholt <eric@anholt.net>