i965/gen6 gs: Convert brw_imm_ud/brw_imm_d to src_reg
i965/gen6 gs: Convert brw_imm_ud/brw_imm_d to src_reg

Same idea as this patch, only for gen6_gs_visitor:

commit 49a938a265f5959c9b558995cc658f80acb6eb18
Author: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>
Date:   Fri Feb 20 12:12:25 2015 -0800
    i965/fs: Use fs_reg for CS/VS atomics pixel mask immediate data

Suggested-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Signed-off-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>
Reviewed-by: Chris Forbes <5dcac483e7a6e31a5cbe89f38cc69dff8ee93dac@ijw.co.nz>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler