drm/omap: Replace drm_dev_unref with drm_dev_put

This patch unifies the naming of DRM functions for reference counting
of struct drm_device. The resulting code is more aligned with the rest
of the Linux kernel interfaces.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>