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 <[e-mail]>
Fixes: bbbe775ec5b5 ("drm: Add support for Amlogic Meson Graphic Controller")
Reviewed-by: Neil Armstrong <[e-mail]>
Signed-off-by: Neil Armstrong <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/meson/meson_plane.c

···
 
 	/* 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 +