media: platform: mtk-mdp: add missed destroy_workqueue in remove

The driver misses destroying wdt_wq when remove like what is done when
probe fails.
Add the missed calls like what is done to job_wq to fix it.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>