drm/msm/sde: Loop through all mixer indexes during handoff

Looping through mixers based on mixer count is incorrrect
as in some targets mixer count less than max mixer indexes
present. Hence loop based on max mixer index.

Change-Id: Id8b2a635b50cfec9b8708ddec89e5517f7764d27
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>