backlight/fbcon: Add FB_EVENT_CONBLANK

The backlight class wants notification whenever the console is blanked
but doesn't get this when hardware blanking fails and software blanking
is used. Changing FB_EVENT_BLANK to report both would be a behaviour
change which could confuse the console layer so add a new event for
software blanking and have the backlight class listen for both.

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>