GBE: don't enable double by default.

GBE: don't enable double by default.

Actually, we don't support double completely currently.
Let's disable it now. This bring a little incompatible point with the 1.2 spec
which doesn't require the kernel to use the following pragma to enable fp64.
 #pragma OPENCL EXTENSION cl_khr_fp64 : enable

If the application wants to try the partially supported double with beignet
under opencl 1.2, the application will still need to add the above pragma.

Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>
Reviewed-by: "Yang, Rong R" <rong.r.yang@intel.com>