GBE: use ISA mad for mad() builtin function.

GBE: use ISA mad for mad() builtin function.

directly map mad() to ISA mad. so mad will have better performance and
less precision loss.

Signed-off-by: Ruiling Song <ruiling.song@intel.com>
Reviewed-by: Zhigang Gong <zhigang.gong@linux.intel.com>