mdss: mdp: Set completion if backlight level is zero

During rapid suspend/resume usecases assertive display is being turned
on with backlight level zero. When on command is received with backlight
level zero driver bails out with out turning on AD and doesn't signal
the client that worker queue has stopped processing which results in a
timeout for on command. Change will ensure that client is signalled if
driver is skipping the processing of AD due to backlight being zero.

Change-Id: Iaa6229f10ce54f44ec64c175f67ef7584ad4c8b2
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>