drm/armada: fix colorkey mode property

The colorkey mode property was not correctly disabling the colorkeying
when "disabled" mode was selected.  Arrange for this to work as one
would expect.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
diff --git a/drivers/gpu/drm/armada/armada_hw.h b/drivers/gpu/drm/armada/armada_hw.h
index 27319a8..345dc4d 100644
--- a/drivers/gpu/drm/armada/armada_hw.h
+++ b/drivers/gpu/drm/armada/armada_hw.h
@@ -160,6 +160,7 @@ enum {
 	CFG_ALPHAM_GRA		= 0x1 << 16,
 	CFG_ALPHAM_CFG		= 0x2 << 16,
 	CFG_ALPHA_MASK		= 0xff << 8,
+#define CFG_ALPHA(x)		((x) << 8)
 	CFG_PIXCMD_MASK		= 0xff,
 };