[Nexus 9 General] gladiac:
Yeah, I was able to reproduce exactly your issue. It panics as soon as I write to "brightness". Additionally, the following messages are written by the kernel:

<6>[ 40.308744] [LED] lp5521_led_multi_color_store , ModeRGB = 1ffffff
<6>[ 40.308779] [LED] multicolor_work_func , ModeRGB = 1
<6>[ 40.308789] [LED] lp5521_led_enable +++
<4>[ 40.343255] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 40.363204] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 40.383041] LP5521-LED 1-0032: [LED] i2c_write_block retry over 2 times
<4>[ 40.413299] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 40.433303] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 40.453034] [LED] I2C_RxData_2 retry over 2 times
<3>[ 40.453048] [LED]i2c_read_block: I2C_RxData fail
<6>[ 40.453056] [LED] lp5521_led_enable reset not ready 0
<4>[ 40.483231] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 40.503211] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 40.523084] LP5521-LED 1-0032: [LED] i2c_write_block retry over 2 times
<4>[ 40.543222] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 40.563259] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 40.583062] LP5521-LED 1-0032: [LED] i2c_write_block retry over 2 times
<4>[ 40.603221] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 40.623349] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 40.643087] LP5521-LED 1-0032: [LED] i2c_write_block retry over 2 times
<4>[ 40.663219] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 40.683233] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 40.703184] LP5521-LED 1-0032: [LED] i2c_write_block retry over 2 times
<4>[ 40.723753] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 40.743217] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 40.763045] LP5521-LED 1-0032: [LED] i2c_write_block retry over 2 times
<6>[ 40.763062] [LED] lp5521_led_enable ---
<4>[ 40.783225] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 40.803431] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 40.823123] LP5521-LED 1-0032: [LED] i2c_write_block retry over 2 times
<4>[ 40.843257] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 40.863225] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 40.883057] LP5521-LED 1-0032: [LED] i2c_write_block retry over 2 times
<4>[ 40.903224] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 40.923236] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 40.943037] LP5521-LED 1-0032: [LED] i2c_write_block retry over 2 times
<4>[ 40.963218] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 40.983265] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 41.003085] LP5521-LED 1-0032: [LED] i2c_write_block retry over 2 times
<4>[ 41.023233] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<4>[ 41.043259] tegra-i2c tegra12-i2c.1: no acknowledge from address 0x32
<3>[ 41.063068] LP5521-LED 1-0032: [LED] i2c_write_block retry over 2 times

Not sure what the cause might be for this.

EDIT: I toyed a litte with the reg-values but that got me nowhere.

We all know that the LED is physically available in a working state before boot (it blinks when the battery is completely empty). Maybe the bootloader disables the LP5521 (flip a register or whatever) which makes the i2c bus unable to communicate with it. That could explain the error messages. Bottom line - I'm not a hardware guy. Chances are that I'm totally wrong with that assumption. What do you guys think?