staging: drm/omap: hold a ref to the bo while waiting for flip

Since the plane hasn't yet taken a reference, we need to hold a
reference while waiting to ensure the backing GEM bo doesn't get
freed from under us.

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