drm/amdgpu/swsmu: add smu11 helper to get manual fan speed (v2)

Will be used to fetch the fan speeds when manual fan mode is
set.

v2: squash in a Coverity fix from Colin Ian King

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