gallium/util: Support POLYGON in u_stream_outputs_for_vertices

u_decomposed_prims_for_vertices cannot support POLYGON, but POLYGON is
trivial to support as a special case directly (since we have the number
of vertices directly).

Fixes aborts in Panfrost in apps using GL_POLYGON.

Fixes: e881aa8c ("gallium/util: Add u_stream_outputs_for_vertices helper")
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Revewied-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit a37822f5)