Revert "drm/msm/dsi-staging: add default brightness property"

This reverts commit 9239fb2e.

Not required on msm-4.14.

Change-Id: I08d0c9c89f69d92fcce72391f96ad953e63a4b6c
Signed-off-by: Wenjun Zhang <wjzhan@codeaurora.org>