backlight: pwm_bl: Drop use of int_pow()

For a fixed small exponent of 3, it is more efficient to simply use
two explicit multiplications rather than calling the int_pow() library
function: Aside from the function call overhead, its implementation
using repeated squaring means it ends up doing four 64x64
multiplications.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/video/backlight/pwm_bl.c

@@ -180,7 +180,8 @@ static u64 cie1931(unsigned int lightness, unsigned int scale)
 	if (lightness <= (8 * scale)) {
 		retval = DIV_ROUND_CLOSEST(lightness * 10, 9033);
 	} else {
-		retval = int_pow((lightness + (16 * scale)) / 116, 3);
+		retval = (lightness + (16 * scale)) / 116;
+		retval *= retval * retval;
 		retval = DIV_ROUND_CLOSEST_ULL(retval, (scale * scale));
 	}