dma-buf: make fence mandatory for dma_resv_add_excl_fence v2

Calling dma_resv_add_excl_fence() with the fence as NULL and expecting
that that this frees up the fences is simply abuse of the internals of
the dma_resv object.

v2: drop the fence pruning completely.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211129120659.1815-4-christian.koenig@amd.com