backlight: don't resume backlight for dark resume

backlight: don't resume backlight for dark resume

For an idle suspend, we set the resume backlight sysfs file which is used during
resume in the kernel. This causes the screen to flash on for a moment during a
dark resume. This change prevents the backlight resume from doing anything
during a dark resume.

BUG=chrome-os-partner:9812
TEST=idle suspend system with dark resume (see backlight doesn't turn on)

Change-Id: Iccd02b99bdd682853fc9b184817c4d43028afcf6
Signed-off-by: Derek Basehore <dbasehore@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/45824
Reviewed-by: Sameer Nanda <snanda@chromium.org>