drm/amdgpu: Enable CP idle interrupts

v1: The interrupts need to be enabled to move to DS clocks.
v2: Don't enable GFX IDLE interrupts if there are no GFX rings.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>