msm: mdss: dsi: Add support for back-light enable gpio

Board platform configurations using apq8084 needs a separate back-light
enable gpio compared to msm8974 where same functionality is piggybacked on
LCD enable gpio. Add support for this back-light enable gpio.

Change-Id: I22f0ab747cb165bbac2055a75fc8ffcc84c79fd2
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>