drm/bridge/synopsys: dsi :remove is_panel_bridge

When using drm_of_panel_bridge_remove() we can simplify the
code and remove is_panel_bridge from dw_mipi_dsi structure.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1506936888-23844-6-git-send-email-benjamin.gaignard@linaro.org