drm/stm: ltdc: Add panel-bridge support

Add the panel-bridge support for both panels & bridges (used by DSI host &
HDMI/LVDS bridges).

Signed-off-by: Philippe CORNU <philippe.cornu@st.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500277223-29553-3-git-send-email-philippe.cornu@st.com