intel/blorp: Use wide formats for nicely aligned stencil clears

In the case where the stencil clear is nicely aligned, we can clear stencil much more efficiently by mapping it as a wide format (say RGBA32_UINT) and blasting out the stencil clear value with a repclear. On Unigine Heaven, this makes one stencil clear go from non-trivial to unnoticeable when looking at per-draw timings.