drm/amdgpu: cleanup amdgpu_ih.c

Cleanup amdgpu_ih.c to be able to handle multiple interrupt rings.

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