The stack trace shows that we should be in agp_find_max() as
called from agp_backend_initialize(). However, agp_find_max()
is __init and its code has already been removed at this point
(since agpgart and intel-agp are separate modules), causing
the kernel to execute random code and eventually oops.  

The patch below works around this by changing agpgart's __init
code & data to normal code & data. Tested, works for me.