If SAL_DISABLE_OPENCL is set we don't want to do any kind of
OpenCL initialization. Put an extra guard in fillOpenCLInfo
(and similar methods in opencl package) to prevent that.

Put the check if OpenCL can be used into one place which checks
SAL_DISABLE_OPENCL and UseOpenCL in configuration.

Change-Id: Icc216d4299d3a7942843117ab9b9411de8075b11
Reviewed-on: https://gerrit.libreoffice.org/30025Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>