msm: mdss: fix DCS backlight control when broadcast mode is enabled

When broadcast mode is enabled, we trigger DCS commands when cmds_tx
is done on controller 1. This call is missing in the path of
backlight control using DCS commands. Add this call to fix DCS
backlight issues for broadcast mode.

Change-Id: I18193375e180a42fb1db42fb1d52a976b660ae1c
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>