msm: mdss: enable under-run interrupt for secondary ctl

In split-display use-cases two ctl paths are active but one acts as
master and other as secondary. Currently under-run interrupt is only
enabled for master ctl. Enable under-run interrupt for secondary ctl
for seamless tracking.

Change-Id: I849419c4077fed384431079952027149c31b33fc
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>