sna: Disable the inline xRGB to ARGB upload conversion

As we have to upload the dirty data anyway, setting the
alpha-channel to 0xff should be free. Not so for firefox-asteroids on
Atom at least.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/sna_blt.c

@@ -1440,6 +1440,9 @@ prepare_blt_put(struct sna *sna,
 						 GXcopy);
 		}
 	} else {
+		if (alpha_fixup)
+			return FALSE; /* XXX */
+
 		if (!sna_pixmap_move_to_cpu(src, MOVE_READ))
 			return FALSE;