drm/ci: Enable CONFIG_BACKLIGHT_CLASS_DEVICE

Dependency for CONFIG_DRM_PANEL_EDP.  Missing this was causing the drm
driver to not probe on devices that use panel-edp.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Tested-by: Helen Koike <helen.koike@collabora.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Link: https://lore.kernel.org/r/20231002164715.157298-1-robdclark@gmail.com


Signed-off-by: Maxime Ripard <mripard@kernel.org>