drm/gem: fold drm_gem_object_put_unlocked and __drm_gem_object_put()

With earlier patch we removed the overhead so now we can lift the helper
into the header effectively folding it with __drm_object_put.

v2: drop struct_mutex references (Daniel)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org> (v1)
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200515095118.2743122-11-emil.l.velikov@gmail.com