Fix ttm_page_alloc.c build breakage

The commit 1e8655f8
    drm/ttm: Fix build on architectures without AGP

looks at TTM_HAS_AGP before it has been set in ttm_bo_driver.h

Move the conditional inclusion of <asm/agp.h> *after* we have included
ttm_bo_driver.h

Signed-of-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>