drm/amd/powerplay: move function thermal_get_temperature to veag20_ppt

the fcuntion thermal_get_temperature will be access SmuMetrics_t data,
the data structure is asic related, so move vega20_ppt to implement.

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>