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>

drivers/gpu/drm/ttm/ttm_page_alloc.c

@@ -40,7 +40,9 @@
 #include <linux/slab.h>
 
 #include <asm/atomic.h>
+#ifdef TTM_HAS_AGP
 #include <asm/agp.h>
+#endif
 
 #include "ttm/ttm_bo_driver.h"
 #include "ttm/ttm_page_alloc.h"