iommu/amd: Fix bug in put_pasid_state_wait

iommu/amd: Fix bug in put_pasid_state_wait

This patch fixes a bug in put_pasid_state_wait that appeared in kernel 4.0
The bug is that pasid_state->count wasn't decremented before entering the
wait_event. Thus, the condition in wait_event will never be true.

The fix is to decrement (atomically) the pasid_state->count before the
wait_event.

Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Cc: stable@vger.kernel.org #v4.0
Signed-off-by: Joerg Roedel <jroedel@suse.de>