drm/panel: panel-simple: Fix proper bpc for ytc700tlag_05_201c

ytc700tlag_05_201c panel support 8 bpc not 6 bpc as per
recent testing in i.MX8MM platform.

Fix it.

Fixes: 7a1f4fa4

 ("drm/panel: simple: Add YTC700TLAG-05-201C")
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210725174737.891106-1-jagan@amarulasolutions.com