drm/ttm: stop pruning fences after wait

This is just abusing internals of the dma_resv object.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211129120659.1815-3-christian.koenig@amd.com