msm: mdss: unstage the pipe if there is z_order mismatch

Unstage the pipe from left mixer if current z_order
does not match with new z_order. MDSS software cannot
unstage such pipes at later point if client has only
called the validate.

Change-Id: Iea914fd1b35b2321f5adb0ac52d739e8a713d386
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>