MAINTAINERS: update AGP tree to point at drm tree

Signed-off-by: Dave Airlie <airlied@redhat.com>

MAINTAINERS

@@ -473,7 +473,7 @@ F:	net/rxrpc/af_rxrpc.c
 
 AGPGART DRIVER
 M:	David Airlie <airlied@linux.ie>
-T:	git git://git.kernel.org/pub/scm/linux/kernel/git/airlied/drm-2.6.git
+T:	git git://people.freedesktop.org/~airlied/linux (part of drm maint)
 S:	Maintained
 F:	drivers/char/agp/
 F:	include/linux/agp*