opencl: fix couple crashes (#12795)

opencl: fix couple crashes (#12795)

* opencl: fix couple crashes

* fix kernel launches failed on devices which do not support
  non-uniform work-groups. When non-uniform work-groups are not
  supported, set `local_work_size` to NULL (= let driver choose the
  work-group sizes). This patch does not cover everything - just the
  cases tested by test-backend-ops.

* fix sub-buffer creation failed due to `cl_buffer_region::origin` not
  being aligned to `CL_DEVICE_MEM_BASE_ADDR_ALIGN`.

* OpenCL: query non-uniform WG sizes only on OpenCL 3.0+