We are about to add a drm_bridge_state that inherits from
drm_private_state which is defined in drm_atomic.h. Problem is,
drm_atomic.h includes drm_crtc.h which in turn includes drm_bridge.h,
leading to "drm_private_state has incomplete type" error.

Let's force all users of the drm_bridge API to explicitly include
drm_bridge.h.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190826152649.13820-2-boris.brezillon@collabora.com