drm/amdgpu: move more interrupt processing into amdgpu_irq.c

Add a callback to amdgpu_ih_process to remove most of the IV logic.

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