drm/msm: Add BACKLIGHT_CLASS_DEVICE as a dependency

One of the SDE files requires BACKLIGHT_CLASS_DEVICE so make sure it
gets selected in the Kconfig.

Change-Id: Ic0dedbadc5bca3ea536d94d54e3683d656bd4aba
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>