drm/msm/sde: fix backlight node for multi display

Update backlight node to generic name to support
multiple display usecase. Remove the separate
backlight file and keep it part of connector
module.

Change-Id: I2c6beb1a27f919c5c201f95133749fdf1d27ddd8
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>