drm/amdgpu: Fix interrupt handling on ih_soft ring

There are no backing hardware registers for ih_soft ring.
As a result, don't try to access hardware registers for read
and write pointers when processing interrupts on the IH soft
ring.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>