msm: mdss: Fix potential NULL pointer dereference in misr set

If control setup is not done and if userspace calls misr set
via metadata ioctl, then there can be a potential NULL pointer
deference as mixer might be NULL. Add a check to validate if
mixer is not NULL.

Change-Id: I92f5778788cfcce6fa01a038b4321222a8804f28
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>