i965: Use last vertex convention for quad provoking vertex on sandybridge

Until we know how hw converts quads to polygon in beginning of
3D pipeline, for now unconditionally use last vertex convention.

Fix glean/clipFlat case.

src/mesa/drivers/dri/i965/brw_context.c

@@ -151,6 +151,13 @@ GLboolean brwCreateContext( int api,
       MIN2(ctx->Const.FragmentProgram.MaxNativeParameters,
 	   ctx->Const.FragmentProgram.MaxEnvParams);
 
+   /* Gen6 converts quads to polygon in beginning of 3D pipeline,
+      but we're not sure how it's actually done for vertex order,
+      that affect provoking vertex decision. Always use last vertex
+      convention for quad primitive which works as expected for now. */
+   if (intel->gen == 6)
+       ctx->Const.QuadsFollowProvokingVertexConvention = GL_FALSE;
+
    if (intel->is_g4x || intel->gen >= 5) {
       brw->CMD_VF_STATISTICS = CMD_VF_STATISTICS_GM45;
       brw->CMD_PIPELINE_SELECT = CMD_PIPELINE_SELECT_GM45;