drm/gma500/psb_intel_lvds: Supply description for 'mode_dev'

drm/gma500/psb_intel_lvds: Supply description for 'mode_dev'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/gma500/psb_intel_lvds.c:635: warning: Function parameter or member 'mode_dev' not described in 'psb_intel_lvds_init'

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Eric Anholt <eric@anholt.net>
Cc: Jesse Barnes <jesse.barnes@intel.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181313.3431493-26-lee.jones@linaro.org