drm/amdgpu/smu: use unified variable smu->is_apu to check apu asic platform

use unified variable smu->is_apu to check apu asic in smu driver.

related patch:
drm/amd/powerplay: bypass dpm_context null pointer check guard for some
smu series

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>