msm: mdss: Check ctl in mdss_mdp_mixer_get before using

Put NULL check for ctl before using in mdss_mdp_mixer_get.

Change-Id: Ia1bc035df46c9e237c244623e61a6802cf0d9110
Signed-off-by: Pawan Kumar <pavaku@codeaurora.org>
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>