disp: msm: sde: avoid backlight updates on enable/disable of TVM

Avoid setting backlight to max/0 during bridge enable/disable
for trusted-vm as the transition between the VMs are expected
to be seamless without any implicit changes to backlight.

Change-Id: Ib828113e49af1be4effea2b71b3106e3cbf401c2
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>