msm: mdss: update mixer pipe_mapped value during overlay_off

The pipe_mapped and pipe_next_map are used to keep track of the
pipes staged in the mixer. The former is used to keep track of the
pipes which are currently used by the hw while the latter keeps track
of the pipes staged for the next frame. During overlay off, this seems
to be off track as only the pipe_next_map would have the updated status
and both would be in sync only after the next commit cycle. This would
cause the mixer to check for invalid pipes during the ctl_stop. Fix the
pipe_mapped state to have the current staged pipes during overlay_off.

Change-Id: I554f753441c54ceedb9dec809effc1119063e59d
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>