drm/msm/sde: use valid device pointer for backlight register

DRM connector does not hold the device pointer on "msm-4.9"
kernel. It is required to pass the device node from kms
to support generic solution. This allows the backlight class
to check the device properties.

Change-Id: I9f62ea7b158f1ef9e5ce886e66ce63ee2cdb4aa7
Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>