drm/amdgpu/acpi: unify ATCS handling (v3)

Treat it like ATIF and check both the dGPU and APU for
the method.  This is required because ATCS may be hung
off of the APU in ACPI on A+A systems.

v2: add back accidently removed ACPI handle check.
v3: Fix incorrect atif check (Colin)
    Fix uninitialized variable (Colin)

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>