When the fdt does not provide a list of brightness states, pwmbl(4)

When the fdt does not provide a list of brightness states, pwmbl(4)
builds a 256 state ramp (i.e. state[i] = i with 0 <= i < 256).

Change the logic to not need to malloc an array of values in this case,
since they are trivially known.

test&ok patrick@