radeonsi: sdma misc + gfx10
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.