backend: refine pow function
backend: refine pow function

remove private array and some unnecessary if check.
convert some if to select. improve about 50% performance

Signed-off-by: rander.wang <5299db4a9bf784920a3e558099f6736bcd8939e6@intel.com>
Tested-by: Yang Rong <78759e37dc7114caed18f74e7515c5cb519dbf6d@intel.com>

wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet