Allow for lazy fence disable on tiling change.

When we disable tiling (for example whenever we free an object to out
userland cache), we stall the gpu so that we can get rid of the fence
register covering its bit of the gtt.

Instead, mark it as invalid and then free it on next use, leading to
less of a gpu stall if any. Leads to some slight performance improvement
on 8xx, 91x and 94x chipsets which are fence constrained.

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