backlight: pwm_bl: Switch to power-of-2 base for fixed-point math

Using a power-of-2 instead of power-of-10 base makes the computations
much cheaper. 2^16 is safe; retval never becomes more than 2^48 +
2^32/2. On a 32 bit platform, the very expensive 64/32 division at the
end of cie1931() instead becomes essentially free (a shift by 32 is
just a register rename).

Signed-off-by: Rasmus Villemoes <[email protected]>
Reviewed-by: Daniel Thompson <[email protected]>
Signed-off-by: Lee Jones <[email protected]>

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