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>