drm/omap: add a comment why locking is missing

unpin_worker() calls omap_framebuffer_unpin() without any locks, which
looks very suspicious. However, both pin and unpin are always called via
the driver's private workqueue, so the access is synchronized that way.

Add a comment to make this clear.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>