Fix clamping when switching terrain providers

[hpinkos]

Fixes #6818

If the tile wasn't loaded yet tile.data.pick would return undefined and data.callback wasn't called. However, we were still setting data.level to the new tile level so on the next frame this whole block was skipped because it thinks it was already called for this LOD. Moving data.level = tile.level into the block where data.callback is called assures that data.level matches the level the callback was called for.

[cesium-concierge]

Thanks for the pull request @hpinkos!

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