i965/gs: Add a flag allowing URB write messages to use a per-slot offset.

i965/gs: Add a flag allowing URB write messages to use a per-slot offset.

This will be used by geometry shaders to implement the EmitVertex()
function, since it requires writing data to a dynamically-determined
offset within the geometry shader's URB entry.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>