drm: add more document for drm_crtc_from_index()

Add a bit more document for function drm_crtc_from_index() to cross
link it with drm_crtc_from_index(), and explain that the function is
useful in vblank code.

While at it, add cross link comment for drm_plane_from_index() as well.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483779131-19935-1-git-send-email-shawnguo@kernel.org