carma-fpga: fix lockdep warning

Lockdep occasionally complains with the message:
INFO: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected

This is caused by calling videobuf_dma_unmap() under spin_lock_irq(). To
fix the warning, we drop the lock before unmapping and freeing the
buffer.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>