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

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 <[email protected]>
Fixes: b86d895524ab ("drm/bridge: Add an ->atomic_check() hook")
Signed-off-by: Boris Brezillon <[email protected]>
Reviewed-by: Laurent Pinchart <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]