"Implement" clGetExtensionFunctionAddress()
"Implement" clGetExtensionFunctionAddress()

This function should not fail if a function entry point cannot be found --
instead we return NULL.

Reviewed-by: Zhigang Gong <e04a7b9b70b1e4c6318cf117dcd1a9056e14b97a@linux.intel.com>

wdv4758h/beignet,wdv4758h/beignet,ignatenkobrain/beignet,wdv4758h/beignet,freedesktop-unofficial-mirror/beignet,zhenyw/beignet,ignatenkobrain/beignet,zhenyw/beignet,zhenyw/beignet,wdv4758h/beignet,freedesktop-unofficial-mirror/beignet,zhenyw/beignet,zhenyw/beignet,ignatenkobrain/beignet,freedesktop-unofficial-mirror/beignet,wdv4758h/beignet,ignatenkobrain/beignet,ignatenkobrain/beignet,freedesktop-unofficial-mirror/beignet