nvidia-graphics-drivers-304 304.135-0ubuntu0.16.04.2 ADT test failure with linux-hwe 4.15.0-24.26~16.04.1

[Khaled El Mously]

Testing failed on:
    amd64: https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-xenial/xenial/amd64/n/nvidia-graphics-drivers-304/20180618_150500_b01a5@/log.gz
    i386: https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-xenial/xenial/i386/n/nvidia-graphics-drivers-304/20180618_150701_b01a5@/log.gz