It was annoying copy-paste happened across OpenCL device constructor, device
enumeration and split kernel checks. Now those areas are using an utility
function which returns pairs of platform and device IDs for devices which are
supported by Cycles and enumeration is happening inside that list.
This makes it so filtering is happening in a single place, so there's no need
to keep 3 different functions in sync.
This commit also fixes a bug with wrong enumeration of devices caused by recent
fixes. Those fixes were in fact wrong and only happened to appear to be working
on laptop with optimus card on Linux. Root of those issues is in fact in bad
Linux driver for optimus cards.
09dc470982