dma-buf: cleanup dma-resv shared fence debugging a bit v2

Make that a function instead of inline.

v2: improve the kerneldoc wording as suggested by Daniel

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602111714.212426-3-christian.koenig@amd.com