i965/gen6/gs: use brw_gs_prog atom instead of brw_ff_gs_prog

i965/gen6/gs: use brw_gs_prog atom instead of brw_ff_gs_prog

This is needed to support user-provided geometry shaders, since the
brw_ff_gs_prog atom in gen6 only takes care of implementing transform feedback
for vertex shaders.

If there is no user-provided geometry shader the implementation falls back to
the original code.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>