draw: revert using correct order for prim decomposition.

This reverts db3dfcfe90.
The commit was correct but we've got some precision problems later in
llvmpipe (or possibly in draw clip) due to the vertices coming in in
different order, causing some internal test failures. So revert for now.
(Will only affect drivers which actually support constant-interpolated
attributes and not just flatshading.)

src/gallium/auxiliary/draw/draw_pt_decompose.h

@@ -3,6 +3,8 @@
    const boolean quads_flatshade_last =      \
       draw->quads_always_flatshade_last;     \
    const boolean last_vertex_last =          \
-      !draw->rasterizer->flatshade_first;
+      !(draw->rasterizer->flatshade &&       \
+        draw->rasterizer->flatshade_first);
+/* FIXME: the draw->rasterizer->flatshade part is really wrong */
 
 #include "draw_decompose_tmp.h"