drm/omap: Replace drm_gem_object_{un/reference} with put,get functions

This patch unifies the naming of DRM functions for reference counting
of struct drm_gem_object. 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>