draw: fix another decompose bug affecting constant interpolated attributes

Previously the first provoking vertex convention would only be used if
flatshading were enabled. No matter how I look at it that cannot be possibly
correct. Maybe the code getting used was somewhat simpler that way at a time
where there weren't constant interpolated attributes, only flatshading...
(Note that all other places including the decomposition macros already do
the same.)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/gallium/auxiliary/draw/draw_pt_decompose.h

    const boolean quads_flatshade_last =      \
       draw->quads_always_flatshade_last;     \
    const boolean last_vertex_last =          \
-      !(draw->rasterizer->flatshade &&       \
-        draw->rasterizer->flatshade_first);
+      !draw->rasterizer->flatshade_first;
 
 #include "draw_decompose_tmp.h"