[AGPGART] Fix modular agpgart ia64 allmodconfig

My previous compat AGP patch broke modular AGPGART.

Test built on;

i386 CONFIG_AGP=y,m
x86_64 CONFIG_AGP=y
ia64 CONFIG_AGP=m

Signed-off-by: Zwane Mwaikambo <zwane@infradead.org>
Cc: Kyle McMartin <kyle@mcmartin.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Jones <davej@redhat.com>