drm/omap: tiler: fix race condition with engine->async

The tiler irq handler uses engine->async value, but the code that sets
engine->async and enables the interrupt does not have a barrier. This
may cause the irq handler to see the old value of engine->async, causing
memory corruption.

Reported-by: Harinarayan Bhatta <harinarayan@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>