msm: mdss: handle used_buf during overlay off with invalid switch state

There might be cases where driver gets the resolution switch
request followed by suspend or adb shell stop. In such cases,
since the ctl parameters are already reconfigured, driver skips
the NULL kickoff during overlay off due to invalid switch state.
This leads to memory leak as the used_bufs are left intact. Fix
by moving the used_bufs to free_list, so that memory can be freed
later and also leaves the buffer list clean.

Change-Id: I80c6ad2dc85da0427ac7ba1c127f0aa044a81c09
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>