drm/zte: Use gem_free_object_unlocked

CMA helpers are struct_mutex free, and so is the zte itself. And
that's the only valid reason for using gem_free_object.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170717151045.4188-3-daniel.vetter@ffwll.ch