drm/amd/powerplay: Fix KASAN user after free on driver unload.

Reusing local handle to initialize BO without resetting it to
NULL is wrong since it causes amdgpu_bo_create_reserved to skip
new BO creation and just reuse the given pointer for pinning.

Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>