drm/i915: fix opregion backlight chip detect and range

BLC_PWM_CTL2 is for 965+ only, so add device model check for
legacy backlight control.

For native backlight control, it maps the backlight value (0~255)
in opregion ASLE[BCLP] to backlight duty cycle (0~max_backlight)
and set into control register.

It also add support for IGD device, which follows opregion spec.

Signed-off-by: Li Peng <peng.li@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>