drm/amd/pm: Avoid writing nulls into `pp_od_clk_voltage`

drm/amd/pm: Avoid writing nulls into `pp_od_clk_voltage`

Calling `smu_cmn_get_sysfs_buf` aligns the
offset used by `sysfs_emit_at` to the current page boundary, which was
previously directly returned from the various `print_clk_levels`
implementations to be added to the buffer position.
Instead, only the relative offset showing how much was written
to the buffer should be returned, regardless of how it was changed
for alignment purposes.

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Ilya Zlobintsev <ilya.zlobintsev@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>