drm: Move a few macros away from drm_crtc.h

Now that there's less stuff in there I noticed that I overlooked them.
Sprinkle some docs over them while at it.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160831160913.12991-2-daniel.vetter@ffwll.ch
diff --git a/include/drm/drm_property.h b/include/drm/drm_property.h
index 30ab289..43c4b6a 100644
--- a/include/drm/drm_property.h
+++ b/include/drm/drm_property.h
@@ -219,6 +219,7 @@
 };
 
 #define obj_to_property(x) container_of(x, struct drm_property, base)
+#define obj_to_blob(x) container_of(x, struct drm_property_blob, base)
 
 /**
  * drm_property_type_is - check the type of a property