msm: mdss: clean any dirty pipes after a successful prepare

After prepare call is made it should be assumed that all pipes that are
needed for this round should already be set. If there are any pipes that
are still dirty after a successful prepare call, it means that they are
not properly referenced in user land. Thus mark them for cleanup.

Change-Id: I6fea19482129223484b8061eb2917aa8987253e5
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>