msm: mdss: fix cursor async left LM staging

commit: cc4d47ad41d136d6d4eae3b6d954be8b1862cdfd
msm: mdss: add async support for hw cursor, introduced the async
support for cursor. To support this, both left and right mixer
of the cursor pipe is staged for dual mixer targets. When the first
cursor update falls on the right LM, only the right mixer is staged,
causing underrun when the cursor position moves to left LM with async
updates. Fix staging of both LM for cursor pipes.

Change-Id: If6f5a35ee1e9456d93a87d150e6a60452b127b2b
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>