Make sure to balance the runtime PM usage counter on driver unbind.

Fixes: 407ccc65 ("[media] davinci: vpif: add pm_runtime support")
Cc: stable@vger.kernel.org      # 3.9
Cc: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Lad Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>