ACPI: video: Simplify __acpi_video_get_backlight_type()

Simplify __acpi_video_get_backlight_type() removing a nested if which
makes the flow harder to follow.

This also results in having only 1 exit point with
return acpi_backlight_native instead of 2.

Note this drops the (video_caps & ACPI_VIDEO_BACKLIGHT) check from
the if (acpi_osi_is_win8() && native_available) return native path.
Windows 8's hardware certification requirements include that there must
be ACPI video bus backlight control, so the ACPI_VIDEO_BACKLIGHT check
is redundant.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>