drm/amdgpu: Fix check for DPM when returning max clock

pp_funcs may not exist, while dpm may be enabled. This change ensures
that KFD topology will report the same as pp_dpm_sclk, as the conditions
for reporting them will be the same.

Otherwise, we may see the issue where KFD reports "100MHz" in topology
as the max speed, while DPM is working correctly.

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>