Bug#870128: libgpuarray: broken autopkgtests

[Rebecca N. Palmer]

Control: tags -1 fixed-upstream

Upstream have a different patch for skipping CUDA-only tests on OpenCL 
(which I haven't tried myself): 
https://github.com/Theano/libgpuarray/commit/f036aef3a425560161de362f390d238f4e7c1721

There are also two real issues in this set of test failures/errors:

- extract_tril/extract_triu - cast across address spaces - fixed 
upstream by 
https://github.com/Theano/libgpuarray/commit/99545026f00e8697bfa5b8fc326f1a808bbe659c

- test_elemwise_f16 add/iadd - wrong rounding mode (half precision is 
low enough for 1-in-last-digit to be out of tolerance) - fixed upstream 
by 
https://github.com/Theano/libgpuarray/commit/5761af18431576e38fa45ff05a0712db84689166

All of these are included in upstream 0.7.6, and given #901487, I 
suggest upgrading rather than cherry-picking.

(This won't stop the tests being D-Uninstallable on non-pocl 
architectures, but given that they wouldn't be usefully runnable there 
anyway (mesa-opencl-icd is available on more architectures but requires 
an AMD/ATI GPU, so won't work on a typical server), they should probably 
stay such.)