drm/msm/sde: change backlight setup to support shared dsi display

Skip backlight setup based on set_backlight op instead of
connector type. This check is more generic and can support connector
that is DSI type but doesn't support backlight control.

Change-Id: Ibc7891a9376e701c55d6d99b08487ef3cd90a283
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>