i965/vec4: Fix mapping attributes

i965/vec4: Fix mapping attributes

This patch reverts 57bab6708f2bbc1ab8a3d202e9a467963596d462, which was
causing issues with ILK and earlier VS programs.

1. brw_nir.c: Revert "i965/vec4/nir: vec4 also needs to remap vs attributes"

   Do not perform a remap in vec4 backend. Rather, do it later when
   setup attributes

2. brw_vec4.cpp: This fixes mapping ATTRx to proper GRFn.

Suggested-by: Kenneth Graunke <kenneth@whitecape.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99391
[jordan.l.justen@intel.com: merge Juan's two patches from bugzilla]
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>