drm: Remove __OS_HAS_AGP

We already express the drm/agp depencies correctly in Kconfig, so we
can rip this remnant from the shared drm core days.

Aside: Pretty much all the #ifdefs in radeon/nouveau could be killed
if ttm would provide dummy functions. I'm not going to volunteer for
that though.

v2: Use IS_ENABLED(CONFIG_AGP) as suggested by Ville

v3: Polish from Ville's review.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com> (v2)
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/drm_bufs.c b/drivers/gpu/drm/drm_bufs.c
index 569064a..f1a204d 100644
--- a/drivers/gpu/drm/drm_bufs.c
+++ b/drivers/gpu/drm/drm_bufs.c
@@ -582,7 +582,7 @@
 	}
 }
 
-#if __OS_HAS_AGP
+#if IS_ENABLED(CONFIG_AGP)
 /**
  * Add AGP buffers for DMA transfers.
  *
@@ -756,7 +756,7 @@
 	return 0;
 }
 EXPORT_SYMBOL(drm_legacy_addbufs_agp);
-#endif				/* __OS_HAS_AGP */
+#endif /* CONFIG_AGP */
 
 int drm_legacy_addbufs_pci(struct drm_device *dev,
 			   struct drm_buf_desc *request)
@@ -1145,7 +1145,7 @@
 	if (!drm_core_check_feature(dev, DRIVER_HAVE_DMA))
 		return -EINVAL;
 
-#if __OS_HAS_AGP
+#if IS_ENABLED(CONFIG_AGP)
 	if (request->flags & _DRM_AGP_BUFFER)
 		ret = drm_legacy_addbufs_agp(dev, request);
 	else