i965/gen6/gs: Use a specific implementation of geometry shaders for gen6.

i965/gen6/gs: Use a specific implementation of geometry shaders for gen6.

In gen6 we will use the geometry shader implementation from gen6_gs_visitor.cpp
and keep the implementation in brw_vec4_gs_visitor.cpp for gen7+. Notice that
gen6_gs_visitor inherits from brw_vec4_gs_visitor so it is not a completely
seprate implementation of geometry shaders.

Also, gen6 does not support multiple dispatch modes, its default operation mode
is equivalent to gen7's SINGLE mode, so select that in gen6 for consistency.

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