drm/amd/powerplay: correct power reading on fiji

Set sampling period as 500ms to provide a smooth power
reading output. Also, correct the register for power
reading.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org