r600g: fix two issues in compressed msaa reading code
r600g: fix two issues in compressed msaa reading code

I've no idea when sample_chan would ever be 4 here, but 4 is most
definitely wrong, array textures have it as 3 as well.

Also the cayman code though unused is obviously wrong.

Signed-off-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

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