glamor: Disable ALPHA8 fbo.

As some platform doesn't support to use ALPHA8 texture as
draw target, we have to disable it. It seems there is no
easy way to check that.

Signed-off-by: Zhigang Gong <zhigang.gong@gmail.com>

glamor/glamor.c

@@ -146,9 +146,11 @@ glamor_create_pixmap(ScreenPtr screen, int w, int h, int depth,
 	return pixmap;
 
     switch (depth) {
+#if 0
     case 8:
         format = GL_ALPHA;
         break;
+#endif
     case 24:
         format = GL_RGB;
         break;