drm/radeon: consolidate redundant macros and constants

After refactoring the _cs logic, we ended up with many
macros and constants that #define the same thing.
Clean'em up.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>