dts: Drop exponentional backlight mapping

This leads to too low brightness (and eventually a completely black screen) in e.g. Phosh as the non-linearity isn't exposed in /sys/class/backlight/backlight-dsi/scale

See https://source.puri.sm/Librem5/linux/-/issues/514

Note that the dropped property is not present in the upstream driver (and unlikely to get added there)