drm/amdgpu/ring: add two interfaces to support r/w registers with kiq

During virtual runtime, need to send command to kiq ring to
read/write GPU registers. Add two interface to support the two
actions.

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