disp: msm: sde: allow backlight device registration for trusted vm

Trusted VM can support dynamic backlight updates through DCS
commands. Allow SDE to register for the backlight device
only if the panel supports DCS backlight update mode.

Note that thermal driver is disabled in trusted vm. So we are
still avoiding registering with msm thermal  driver. The
backlight updates are only supported through backlight driver
provided sysfs nodes.

Change-Id: I9cd41d473445387c8bc9048d7e565d49124a36b7
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>