drm/panel: panel-simple: Fix proper bpc for AM-1280800N3TZQW-T00H

AM-1280800N3TZQW-T00H panel support 8 bpc not 6 bpc as per
recent testing in i.MX8MM platform.

Fix it.

Fixes: bca684e6

 ("drm/panel: simple: Add AM-1280800N3TZQW-T00H")
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211111094103.494831-1-jagan@amarulasolutions.com