drm/i915/gem: Reinitialise the local list before repeating

As we may start the loop again, we require our local list of i915_vma
we've processed to be reinitialised.

Fixes: aa5e4453 ("drm/i915/gem: Try to flush pending unbind events")
Closes: https://gitlab.freedesktop.org/drm/intel/issues/731


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205132912.606868-1-chris@chris-wilson.co.uk