i965: Implement GL_PRIMITIVES_GENERATED with non-zero streams.

i965: Implement GL_PRIMITIVES_GENERATED with non-zero streams.

So far we have been using CL_INVOCATION_COUNT to resolve this query but this
is no good with streams, as only stream 0 reaches the clipping stage. Instead
we will use SO_PRIM_STORAGE_NEEDED which can keep track of the primitives sent
to each individual stream.

Since SO_PRIM_STORAGE_NEEDED is related to the SOL stage and according to
ARB_transform_feedback3 we need to be able to query primitives generated in
each stream whether transform feedback is active or not what we do is to
enable the SOL unit even if transform feedback is not active but disable all
output buffers in that case. This effectively disables transform feedback
but permits activation of statistics enabling SO_PRIM_STORAGE_NEEDED even
when transform feedback is not active.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>