[media] mtk-mdp: fix double mutex_unlock

Fix smatch error:

media-git/drivers/media/platform/mtk-mdp/mtk_mdp_vpu.c:100 mtk_mdp_vpu_send_msg() error: double unlock 'mutex:&ctx->mdp_dev->vpulock'

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>