drm/radeon: add rdev in ring struct

Retrieving radeon device struct from ring struct will be
used in next patch where debugfs's show function can only pass
one private data pointer.

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