drm/msm: remove msm_drm_private::bridges field

As all output devices have switched to devm_drm_bridge_add(), we can
drop the bridges array from struct msm_drm_private.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/561641/


Signed-off-by: Rob Clark <robdclark@chromium.org>