msm: mdss: Support updates from both the mixers on splitdisplay

Support updates in both mixers when running ARGC,PCC test
(destination side) on split display.Allow the dualpipe case
to update simultaneously on both right and left mixers by using
"ctl" structures's pointers to left and right mixers instead of
"split ctl" structures.

Change-Id: Ic5fb291802af35f08870f2f121c5aff93d0b993e
Signed-off-by: Mukesh Jha <cmjha@codeaurora.org>