brw/gs: Rename brw_gs_compile to brw_ff_gs_compile

The current brw_gs_compile struct is used to compile geometry shaders
that implement fixed-functionality behaviours (tesselation of quads on
Gen 4-5 and transform feedback on Gen 6).  We're going to need a
different structure when compiling user-provided geometry shaders.  So
rename brw_gs_compile to brw_ff_gs_compile to emphasize that it is
only used for fixed functionality.
WIP: add GS visitors