drm/meson: Correct OSD1 global alpha value

drm/meson: Correct OSD1 global alpha value

[ Upstream commit 955d8719516428357dbda7e1f1d146cf6ca6ae41 ]

VIU_OSD1_CTRL_STAT.GLOBAL_ALPHA is a 9 bit field, so the maximum
value is 0x100 not 0xff.

This matches the vendor kernel.

Signed-off-by: Stuart Menefy <stuart.menefy@mathembedded.com>
Fixes: e5a2f708e761 ("drm: Add support for Amlogic Meson Graphic Controller")
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220908155103.686904-1-stuart.menefy@mathembedded.com
Signed-off-by: Sasha Levin <sashal@kernel.org>