drm/i915/gem: Optimistically prune dma-resv from the shrinker.

As we shrink an object, also see if we can prune the dma-resv of idle
fences it is maintaining a reference to.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201223122051.4624-2-chris@chris-wilson.co.uk