drm/radeon: check bo_va->bo is non-NULL before using it

The call to radeon_vm_clear_freed might clear bo_va->bo, so
we have to check it before dereferencing it.

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>