drm/amdgpu: Call amdgpu_ucode_fini_bo in amd_powerplay.c

make it symmetric with amdgpu_ucode_init_bo in amd_powerplay.c

refine the "commit b22558bb4ff8fc9fe925222f90297d7a03a5fb20"

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>