drm/amdgpu: Remove unnecessary NULL check

A static checker pointed out, that bo_va->base.bo was already derefenced
earlier in the same scope. Therefore this check is unnecessary here.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Fixes: 50661eb1

 ("drm/amdgpu: Auto-validate DMABuf imports in compute VMs")
Reviewed-by: Kent Russell <kent.russell@amd.com>
Signed-off-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>