backlight: qcom-wled: Provide enabled_strings default for WLED 4 and 5

Only WLED 3 sets a sensible default that allows operating this driver
with just qcom,num-strings in the DT; WLED 4 and 5 require
qcom,enabled-strings to be provided otherwise enabled_strings remains
zero-initialized, resulting in every string-specific register write
(currently only the setup and config functions, brightness follows in a
future patch) to only configure the zero'th string multiple times.

Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211115203459.1634079-8-marijn.suijten@somainline.org