tools/power/turbostat: Improve probe_platform_features() logic

AMD/Hygon platforms that don't have RAPL use 'amd_features' to describe
the platform features. Unknown Intel platforms use 'default_features' to
describe the platform features.

As none of the platform feature is set for 'amd_features' or
'default_features', there is no need to maintain both of them.

Remove 'amd_features' structure and improve the logic in
probe_platform_features().

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>