drm/amdgpu: Polish SQ IH.

Switch to using reg fields defines istead of magic values.
Add SH_ID and PRIV fields reading for instr. and err cases.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>