i965: Rename brw_gs{,_emit}.[ch] to brw_ff_gs{,_emit}.[ch].

i965: Rename brw_gs{,_emit}.[ch] to brw_ff_gs{,_emit}.[ch].

The brw_gs.[ch] and brw_gs_emit.c source files contain code for
emulating fixed-function unit functionality (VF primitive decomposition
or SOL) using the GS unit.  They do not contain code to support proper
geometry shaders.

We've taken to calling that code "ff_gs" (see brw_ff_gs_prog_key,
brw_ff_gs_prog_data, brw_context::ff_gs, brw_ff_gs_compile,
brw_ff_gs_prog).  So it makes sense to make the filenames match.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Matt Turner <mattst88@gmail.com>
Acked-by: Jason Ekstrand <jason.ekstrand@intel.com>
Acked-by: Iago Toral Quiroga <itoral@igalia.com>