drm/radeon: document radeon_ring.c (v4)

Adds documentation to most of the functions in
radeon_ring.c

v2: adjust per Christian's suggestions
v3: adjust per Christian's latest patches
v4: adjust per my latest changes

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