msm: mdss: Change ctl allocation

Need to assign ctl that is consistent with the type of mixer
allocated. Specifically for writeback mixers so that it can be
separated from interface ctls.

Change-Id: I7ae7316685286eab6202b91d8347bfd128b0a0b3
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>