msm: vidc: fix iommu attach failures at end of playback

The power_enabled flag should be reset to zero
only after iommu domains are detached.
By resetting the flag to zero earlier when the
iommu domains are still attached, unmap
calls made  in v4l2 driver will try to
attach the iommu domains again, resulting in
error messages.

Change-Id: Ifb0e532a472864ab89865fe85c4f396455a37641
Signed-off-by: Arun Menon <avmenon@codeaurora.org>