drm/bridge: tfp410: fix use of cancel_delayed_work_sync

We use delayed_work in HPD handling, and cancel any scheduled work in
tfp410_fini using cancel_delayed_work_sync(). However, we have only
initialized the delayed work if we actually have a HPD interrupt
configured in the DT, but in the tfp410_fini, we always cancel the work,
possibly causing a WARN().

Fix this by doing the cancel only if we actually had the delayed work
set up.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190610135739.6077-2-tomi.valkeinen@ti.com