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: NKevin Wang <kevin1.wang@amd.com>
Reviewed-by: NHuang Rui <ray.huang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>