drm/amdgpu/pm: Use macro definitions in the smu IH process function

Replace the hard-coded numbers with macro definition

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>