drm/vc4: remove bridge from driver internal structure

With a call to drm_of_panel_bridge_remove() we could remove
the bridge without store it in vc4_dpi internal driver structure.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1506936888-23844-5-git-send-email-benjamin.gaignard@linaro.org