GPU tests started failing on K40

[inducer]

PYOPENCL_TEST=nvi:k40 python test_fmm.py 'test_fmm_completeness(cl._csc, 3, 50000, 40000, "", p_normal, p_normal, None)' 

seems to fail somewhat reliably. Strangely, this happens with a fairly ancient set of versions: 88bdb3ad016d89565e98a87c9e7bc4fe44e9a8f1 for boxtree, and f999323804b6df44abf7da1181e66fed831f86f7 for pyopencl. It happens in the same fashion with newer code. The timing coincides with an Nvidia driver upgrade on those machines. I've tried rolling back to an older driver, but I can't quite reconstruct what version we were on.

cc @mattwala

[inducer]

Affected drivers are 370.x and 375.x. 367.x is what we had before.

[inducer]

Restoring to 367.x seems to have fixed things.