drm/amdgpu/swsmu: drop get_fan_speed_percent (v2)

No longer needed as we can calculate it based on
the fan's max rpm.

v2: rework code to avoid possible uninitialized
variable use.

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