backlight: pwm_bl: Move the checks for initial power state to a separate function

Move the checks to select the initial state for the backlight to a new
function and document the checks we are doing.

With the separate function it is going to be easier to fix or improve the
initial power state configuration later and it is easier to read the code.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>