i965: Rename brw_vec4_gs.[ch] to brw_gs.[ch].

i965: Rename brw_vec4_gs.[ch] to brw_gs.[ch].

These source files support actual geometry shaders, so using "gs" for
the name makes a lot of sense.  We're going to be adding SIMD8 geometry
shader support as well, at which point "vec4_gs" will be a misnomer.

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>