it looks like the suspend sequence depends on the dt node sequence, and
we are putting display-subsystem dt node above spi dt node, so it would
be earlier in the device list, then got suspended later than spi device.
the pwm backlight and cros_ec_spi pwm are very interesting, not only
about suspend dependency... if we unbind cros_ec_spi pwm, the pwm
backlight would still hold a reference to it, and crash the kernel later.