drm/meson: Correct OSD1 global alpha value

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: bbbe775 ("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

Author: Stuart Menefy

drivers/gpu/drm/meson/meson_plane.c

@@ -170,7 +170,7 @@ static void meson_plane_atomic_update(struct drm_plane *plane,
 
 	/* Enable OSD and BLK0, set max global alpha */
 	priv->viu.osd1_ctrl_stat = OSD_ENABLE |
-				   (0xFF << OSD_GLOBAL_ALPHA_SHIFT) |
+				   (0x100 << OSD_GLOBAL_ALPHA_SHIFT) |
 				   OSD_BLK0_ENABLE;
 
 	priv->viu.osd1_ctrl_stat2 = readl(priv->io_base +