drm/amdgpu/swsmu/renoir: simplify sensor handling (v2)

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

v2: use proper enum

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>