lang/beignet: make OpenCL 2.0 optional on amd64

This is mainly to aid debugging OpenCL 2.0 issues and help testing
OpenCL 1.2 without relying on kernel translating 32bit DRM ioctls.

PR: 217771
Approved by: maintainer timeout (2 months)
MFH: 2017Q2 (requires r437953), 2017Q3
Differential Revision: https://reviews.freebsd.org/D11377