i965/gen6/gs: Add initial implementation for a gen6 geometry shader visitor.

i965/gen6/gs: Add initial implementation for a gen6 geometry shader visitor.

Geometry shaders in gen6 are significantly different from gen7+ so it is better
to have them implemented in a different file rather than adding gen6 branching
paths all over brw_vec4_gs_visitor.cpp.

This commit adds an initial implementation that only handles point output, which
is the simplest case.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>