drm/omap: Fix release of refill engine

During asynchronous refills, we don't wait for the refill to
finish.  However, we cannot release the engine back to the idle
list until it has actually completed the refill operation.  The
engine release will now be done in the IRQ handler, but only
for asynchronous refill operations.

Synchronous refills will continue to release the engine after they
unblock from waiting on the refill.

v2: Fixed review comments on async variable and bool type

Signed-off-by: Andy Gross <andy.gross@ti.com>
Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>