i965: Port push constant code to genxml.

i965: Port push constant code to genxml.

The following states are ported on this patch:
   - gen6_gs_push_constants
   - gen6_vs_push_constants
   - gen6_wm_push_constants
   - gen7_tes_push_constants

v2:
   - Use helper to setup brw_address (Kristian)
v3:
   - Do not use macro for upload_constant_state (Ken)
   - Do not re-declare MOCS macro (Ken)
v4: (by Ken)
   - Drop more dead code, change brw->gen checks to GEN_GEN, style nits

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>