i965: Fix gl_VertexID when there are no other vertex inputs.

brw_emit_vertices contains special case logic to handle the case where
a vertex shader doesn't read any inputs.  This special case logic was
incorrectly activating in the case were the only vertex input is
gl_VertexID.  As a result, if a shader used gl_VertexID but used no
other inputs, then all vertices got a gl_VertexID of zero.

Fixes oglconform test "ubo-usage advanced.transform_feedback".

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

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

@@ -571,6 +571,8 @@ static void brw_emit_vertices(struct brw_context *brw)
 
    brw_emit_query_begin(brw);
 
+   nr_elements = brw->vb.nr_enabled + brw->vs.prog_data->uses_vertexid;
+
    /* If the VS doesn't read any inputs (calculating vertex position from
     * a state variable for some reason, for example), emit a single pad
     * VERTEX_ELEMENT struct and bail.
@@ -578,7 +580,7 @@ static void brw_emit_vertices(struct brw_context *brw)
     * The stale VB state stays in place, but they don't do anything unless
     * a VE loads from them.
     */
-   if (brw->vb.nr_enabled == 0) {
+   if (nr_elements == 0) {
       BEGIN_BATCH(3);
       OUT_BATCH((_3DSTATE_VERTEX_ELEMENTS << 16) | 1);
       if (intel->gen >= 6) {
@@ -642,8 +644,6 @@ static void brw_emit_vertices(struct brw_context *brw)
       ADVANCE_BATCH();
    }
 
-   nr_elements = brw->vb.nr_enabled + brw->vs.prog_data->uses_vertexid;
-
    /* The hardware allows one more VERTEX_ELEMENTS than VERTEX_BUFFERS, presumably
     * for VertexID/InstanceID.
     */