msm: mdss: unstage pipe from right mixer in error case

Unstage pipe from right mixer also if pipe staging
fails due to any error. This removes the invalid pipe
usage on right mixer when source split is enabled.

Change-Id: Iac8365025630b71808ea3e0f6ec849964ef52b9c
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>