i965/gs: Do prog_data setup and other calculations in brw_compile_gs

i965/gs: Do prog_data setup and other calculations in brw_compile_gs

This commit moves the large pile of setup calculations we have to do for
geometry shaders out of brw_gs_emit and into brw_compile_gs.  This has a
couple of nice implications.  First, it's less work that the caller of
brw_compile_gs has to do.  Second, it's consistent with the vertex and
fragment stages.  Finally, it allows us to put brw_gs_compile back behind
the API boundary where it belongs.

v2 (Jason Ekstrand):
 - Pull the changes to use nir info into a separate patch
 - Put brw_gs_compile into brw_shader.h rather than brw_vec4_gs_visitor.h
   so that we can use it for scalar GS.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>