msm: mdss: clean up overlay resources properly when display is off

When display is turned off, the clocks are already turned off and
unprepared.  Earlier, when unsetting the overlay resource, we were
trying to re-enable the unprepared clocks.

Change-Id: I36e79e4a0bdbbe35f4474527a09a4da59cbec96c
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>