drm: clarify adjusted_mode documentation for bridges

This patch clarifies the adjusted_mode documentation
for bridges.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180409152427.12449-1-philippe.cornu@st.com