drm/radeon: rework ring function handling

Give the ring functions a separate structure and let the asic
structure point to the ring specific functions. This simplifies
the code and allows us to make changes at only one point.

No change in functionality.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>