radeonsi: sdma misc + gfx10

Merged Pierre-Eric Pelloux-Prayer requested to merge pepp/mesa:radeonsi_sdma_misc into master

The 2 first commits are cleanups of the sdma code to match the documentation and PAL code.

The 3rd one temporarily disables SDMA for gfx10 since it causes hangs (e.g: #1907 (closed) or https://bugs.freedesktop.org/show_bug.cgi?id=111481). It might impact performance but the hangs are quite annoying for the users. As soon as the underlying issue is fixed, SDMA support will be restored.

Edited by Pierre-Eric Pelloux-Prayer

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