drm/omap: call drm_put_dev directly in ->remove

Again omap already sets the driver data pointer to the drm_device.

Also drop the driver unregister call, that should be (and already is)
done in the module unload hook.

Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>