tu: Support multisample image clears

tu: Support multisample image clears

We may need shader workarounds for some formats, but for now this seems
to work at least as well as the gmem path for clearing multisample
attachments. And soon we'll start calling this even on the gmem path,
since we leave the final decision of whether to use sysmem or not up
till the end, so we can't have it assert or otherwise working tests
would assert.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3713>