drm: panel: simple: Fix bpc for LG LB070WV8 panel

The LG LB070WV8 panel incorrectly reports a 16 bits per component value,
while the panel uses 8 bits per component. Fix it.

Fixes: dd015002 ("drm/panel: simple: Add support for LG LB070WV8 800x480 7" panel")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200711225317.28476-1-laurent.pinchart+renesas@ideasonboard.com