Disable A8 texture format for GLES2.

As PVR's GLES2 implementation doesn't support A8 texture as
rendering target, we disable it for now.

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

glamor/glamor_utils.h

@@ -240,13 +240,10 @@ static inline void
 gl_iformat_for_depth(int depth, GLenum * format)
 {
 	switch (depth) {
+#ifndef GLAMOR_GLES2
 	case 8:
 		*format = GL_ALPHA;
 		break;
-#if 0
-	case 24:
-		*format = GL_RGB;
-		break;
 #endif
 	default:
 		*format = GL_RGBA;