staging: drm/omap: add a workqueue

Add a workqueue for omapdrm driver, which is needed for at least a
couple things currently: (1) moving omap_gem_roll() to a non-atomic
context, (2) synchronizing page flips w/ DSS scanout related irq's
(in particular not unmapping previous buffer until DSS finishes
scanout).

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