drm/amdgpu: implement jpeg ring functions

Implement all ring functions needed for jpeg ring

v2: remove unnecessary mem read function.

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