drm/bridge: Fix a NULL pointer dereference in drm_atomic_bridge_chain_check()

drm_atomic_bridge_chain_check() callers can pass a NULL bridge. Let's
bail out before dereferencing the bridge pointer when that happens.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: b86d8955

 ("drm/bridge: Add an ->atomic_check() hook")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200107113031.435604-1-boris.brezillon@collabora.com