staging: drm/omap: move where DMM driver is registered

Not sure what triggered the change in behavior, but seems to
result in recursively acquiring a mutex and hanging on boot.  But
omap_drm_init() seems a much more sane place to register the
driver for the DMM sub-device.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>