ACPI: video: create "brightness_switch_enabled" modparam

Introduce new module parameter for brightness control.
"brightness_switch_enabled" is set by default which means
nothing changes upon brightness switch events.

When "brightness_switch_enabled" is cleared via
"echo 0 > /sys/module/video/parameters/brightness_switch_enabled",
ACPI will not try to change the brightness level any more.

Either X will take charge of this or users can change the brightness level
by poking /sys/class/backlight/acpi_videoX/...

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>