How does integrated displays controlled by the computer?
Not sure what this question is about. Yes, brightness is controlled by changing intensity of backlit LED, which requires a separate controller (usually interfaced with mainboard via a serial link, I2C or SMBus). Yes, proximity sensors are also INDIVIDUAL devices that must be interfaced with mainboard, again via I2C or SMBus. So the mainboard, if you want it to control brightness and turn it completely off via proximity sensor, must have a dedicated CONTROLLER in PC I/O space, and a corresponding driver to let application software to access this controller. Typical consumer "mobos" usually don't have this, although many chipsets do have a channel to do this kind of job, usually via a dedicated peripheral "super I/O chip" connected via the standard 4-bit-wide LPC bus. Consumer mainboards usually don't expose this functionality to user space.

Many stand-alone (VESA-compliant) displays have a side-band communication channel called DDC, which, if configured/used, and your video card supports it, can control brightness and all other settings in the display.

If your intent is to use a cheap consumer mainboard in a kiosk-like application, you need to consider the special class of "embedded mainboards", PC-cards, etc, which are, unfortunately, quite more expensive.