drm/amdgpu: remove unused variable from struct amdgpu_bo

Fixes: 62914a99

 ("drm/amdgpu: Use mmu_interval_insert instead of hmm_mirror")
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>