drm/ttm: Fix build on architectures without AGP

Make inclusion of <asm/agp.h> conditional on TTM_HAS_AGP.  The use
of the functions declared in it is already conditional.

Reported-by: Geert Stappers <stappers@stappers.nl>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Geert Stappers <stappers@stappers.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>