dma-buf: Remove unneeded stubs around sync_debug interfaces

The sync_debug.h header is internal, and only used by
sw_sync.c. Therefore, SW_SYNC is always defined and there
is no need for the stubs. Remove them and make the code
simpler.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180504180037.10661-1-ezequiel@collabora.com