drm/amdgpu/pm: use read_sensor API to get temperature

Rather than using the amdgpu_dpm_get_temperature.  Both
provide access to the temperature.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>