drm: remove transient drm_gem_object_put_unlocked()

As of last commit, all the drivers have been updated away from the
_unlocked helper. As such we can now remove the transient #define.

v2: keep sed and #define removal separate

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org> (v1)
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200515095118.2743122-39-emil.l.velikov@gmail.com