[PATCH] Adding backlight support for the i915 driver.

This  patch exposes the backlight support  via a sysctl:

set the backlight to 10%:

# sysctl hw.dri.0.i915_backlight=10

hw.dri.0.i915_backlight: 25 -> 10

set the backlight to 50%:

# sysctl hw.dri.0.i915_backlight=50

hw.dri.0.i915_backlight: 10 -> 50

decrease  the current backlight value:

# sysctl hw.dri.0.i915_backlight=-1000

hw.dri.0.i915_backlight: 50 -> 43

increment the current backlight value:

# sysctl hw.dri.0.i915_backlight=1000

hw.dri.0.i915_backlight: 43 -> 51

# sysctl hw.dri.0.i915_backlight=1000

hw.dri.0.i915_backlight: 51 -> 60

I am running this path on for about a week without issue.

This path can be found at:
https://github.com/maurizio-emmex/i915_backlight_freebsd

I thank Elizabeth Myers, elizabeth at interlinked dot me, for the idea of
adding the backlight support for the i915 driver and for the original patch.

Regards,
Maurizio