drm/amdgpu: Optimize checking ras supported

Using "is_app_apu" to identify device in the native
APU mode or carveout mode.

Signed-off-by: Stanley.Yang <Stanley.Yang@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>