Improve backlight control

This commit fixes backlight support for several platforms.

Except on recent machines supporting the IGD OpRegion specification,
backlight control is rather platform specific.  In some cases, we can
program the native backlight control regsiters directly without any
trouble.  On others, we need to use the legacy backlight control
register.  On still others, we need a combination of the two.  And on
some platforms, none of the above will work, so we go through the
kernel backlight interface, which provides a platform specific driver
for backlight control.

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