drm/amdgpu: Drop unused variable and statement

Even though 'smu8_smu' is declared, it is not used after below statement.

   smu8_smu = hwmgr->smu_backend;

So 'unused variable' could be safely removed
to stop warning message as below:

   drivers/gpu/drm/amd/amdgpu/../powerplay/smumgr/smu8_smumgr.c:180:22:
   warning: variable ‘smu8_smu’ set but not used
   [-Wunused-but-set-variable]

   struct smu8_smumgr *smu8_smu;
             ^
Signed-off-by: Austin Kim <austindh.kim@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>