dma-buf: Do not build debugfs related code when !CONFIG_DEBUG_FS

There is no point in compiling in the list and mutex operations which are
only used from the dma-buf debugfs code, if debugfs is not compiled in.

Put the code in questions behind some kconfig guards and so save some text
and maybe even a pointer per object at runtime when not enabled.

Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Christian König <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-dev@igalia.com
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240328145323.68872-1-tursulin@igalia.com