[CUDA] probe and initialize all devices
[CUDA] probe and initialize all devices

We ask the CUDA driver API about the number of available device. This
allows running pocl with CUDA support enabled regardless of whether CUDA
devices are available or not.

Additionally, in the initialization phase, we actually get a different
device each time rather than always the first.

Signed-off-by: Giuseppe Bilotta <6010d7f2c049984f25cd19568d4ace2cc46b0817@gmail.com>

Oblomov/pocl,franz/pocl,rjodin/pocl,pocl/pocl,franz/pocl,rjodin/pocl,Oblomov/pocl,franz/pocl,Oblomov/pocl,rjodin/pocl,rjodin/pocl,pocl/pocl,pocl/pocl,pocl/pocl,franz/pocl,Oblomov/pocl,pocl/pocl,Oblomov/pocl,franz/pocl