drm/amd/powerplay: remove unnecessary checking in smu_hw_fini

This patch removes unnecessary NULL pointer checking in smu_hw_fini, because
kfree is able to handle NULL pointer case.

Suggested-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>