drm/amd/powerplay: initialize the soft_regs offset in struct smu7_hwmgr

This could lead to mclk dpm problems on some boards.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Ack-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>