drm/amdgpu: cleanup amdgpu_ih_process a bit more

Remove the callback and call the dispatcher directly.

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