drm/ttm: Remove TTM_HAS_AGP

It tries to do fancy things with excluding agp support if ttm is
built-in, but agp isn't. Instead just express this depency like drm
does and use CONFIG_AGP everywhere.

Also use the neat Makefile magic to make the entire ttm_agp_backend
file optional.

v2: Use IS_ENABLED(CONFIG_AGP) as suggested by Ville

v3: Review from Emil.

v4: Actually get it right as spotted by 0-day.

Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1459337046-25882-1-git-send-email-daniel.vetter@ffwll.ch

include/drm/ttm/ttm_bo_driver.h

@@ -1030,8 +1030,7 @@ extern pgprot_t ttm_io_prot(uint32_t caching_flags, pgprot_t tmp);
 
 extern const struct ttm_mem_type_manager_func ttm_bo_manager_func;
 
-#if (defined(CONFIG_AGP) || (defined(CONFIG_AGP_MODULE) && defined(MODULE)))
-#define TTM_HAS_AGP
+#if IS_ENABLED(CONFIG_AGP)
 #include <linux/agp_backend.h>
 
 /**