drm/msm/dsi-staging: add dcs backlight support to dsi driver

Add support to send dcs commands to control the
backlight for panels that use this method
to update the brightness level.

Change-Id: Ie453aee11c6b0570c0d76244c0617c766c356b43
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>