dma-buf: finally make the dma_resv_list private v2

Drivers should never touch this directly.

v2: drop kerneldoc for now internal handling

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220321135856.1331-2-christian.koenig@amd.com