gallivm: fix out-of-bounds behavior for fetch/ld

gallivm: fix out-of-bounds behavior for fetch/ld

For d3d10 and ARB_robust_buffer_access_behavior, we are required to return
0 for out-of-bounds coordinates (for which we can just enable the code already
there was just disabled). Additionally, also need to return 0 for
out-of-bounds mip level and out-of-bounds layer. This changes the logic
so instead of clamping the level/layer, an out-of-bound mask is computed
instead in this case (actual clamping then can be omitted just like with
coordinates, since we set the fetch offset to zero if that happens anyway).

Reviewed-by: Zack Rusin <zackr@vmware.com>