drm/amdgpu/swsmu/vangogh: simplify sensor handling

Just query the metrics table directly rather than going through
an extra level of functions.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>