drm/amdgpu: fix unused variable

SOC15_WAIT_ON_RREG's return value needn't always been handled by
caller. new design is to fix this kind of unused variable.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>