GBE: improve precision of rootn
GBE: improve precision of rootn

Signed-off-by: Lv Meng <2df664ee4255226675c7b29e26307ac8da3842dc@intel.com>

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