gallium/util: Add u_stream_outputs_for_vertices helper

gallium/util: Add u_stream_outputs_for_vertices helper

This u_prim.h helper determines the number of outputs for stream output,
given a particular primitive type and a vertex count. This is useful for
statically calculating sizes of stream output buffers (i.e. when there
is no geometry/tessellation shader in use).

This helper will be used in Panfrost's transform feedback
implementation, as you can probably guess since why else would I be
submitting it....

See also dEQP's getTransformFeedbackOutputCount routine.

v2: Simplify definition using new helpers, which also extends to non-ES2
primitive types (Eric).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>