radeonsi: clean up CP DMA emit code
radeonsi: clean up CP DMA emit code

Unify the clear and copy paths, clean up the definitions.
It looks more like a rework. It's a preparation for GDS support,
which might or might not come.

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler