drm/amdgpu: Declare Unpin BO api as static

Fixes warning report from kernel test robot

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>