i965/gen6/gs: Buffer PSIZ/flags vertex data in gen6_gs_visitor

i965/gen6/gs: Buffer PSIZ/flags vertex data in gen6_gs_visitor

Since geometry shaders can alter the value of varyings packed in the first
output VUE slot (PSIZ), we need to buffer it together with all the other
vertex data so we can emit the right value for each vertex when we do the
URB writes.

This fixes the following piglit test in gen6:
tests/spec/glsl-1.50/execution/redeclare-pervertex-out-subset-gs.shader_test

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>