msm: mdss: ensure retire fences are signaled during panel off

When turning off command mode panel, if retire fences are still active
and suspend sequence goes through some fences may remain active.
If fences remain active command panel off sequence would stop signaling
vsyncs and would throw off the timeline. To fix this clear the retire
timeline when turning panel off.

Change-Id: I5135eaf27b4b206e9d6f61db340360d3d11719fd
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>