drm/amd/pp: Remove unneeded void * casts in cz_hwmgr.c/cz_smumgr.c

Removes unneeded void * casts for the following pointers:
hwmgr->backend
hwmgr->smu_backend

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