drm/omap: Fix memory leak in omap_gem_op_async

In omap_gem_op_async(), if a waiter is not added to the wait list, it needs to
be free'd in the function itself.

Make sure we free the waiter for this case.

Signed-off-by: Subhajit Paul <subhajit_paul@ti.com>
Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>