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