msm: vidc: Do not power off Venus if already powered off

If the driver tries to unload the firmware after it has
gone in to power collapse, there is no check in the power_off
function to see if it's already powered off or not. Hence
we would end up disabling the regulators and clocks again,
thereby messing up the regulator count and also disable the
irq, without enabling it. This will impact subsequent video
sessions and might need a device restart to fix things.

Change-Id: Icffa7e53d358beb34df85b0184c9f526910f8220
Signed-off-by: Arun Menon <avmenon@codeaurora.org>