drm/tinydrm: Use gem_free_object_unlocked

tinydrm doesn't use dev->struct_mutex and therefore has no need to use
gem_free_object.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322105133.11211-2-daniel.vetter@ffwll.ch