drm/amdgpu: Modify for_each_inst macro

Modify it such that it doesn't change the instance mask parameter.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Acked-by: Victor Skvortsov <victor.skvortsov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>