Fix AGP compile on non-x86 architectures

AGP shouldn't use "global_flush_tlb()" to flush the AGP mappings, that i
spurely an x86'ism.  The proper AGP mapping flusher that should be used
is "flush_agp_mappings()", which on x86 obviously happens to do a global
TLB flush.

This makes AGP (or at least the config _I_ happen to use) compile again
on ppc64.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>