dma: ipu: Drop unused spinlock

I was checking why this spinlock was never initialized, but it turns
out it's not used anywhere, so we can drop it.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Dan Williams <djbw@fb.com>