dma-buf: cleanup reservation_object_init/fini

They are not used that often and certainly not in a hot path.
Make them normal functions instead of an inline.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/314480/