i965/blorp: Prepare render target write for gen8

i965/blorp: Prepare render target write for gen8

v2 (Ken): Use payload directly instead of retyping it into vec8.
          Drop the implied header, it isn't used for gen6+ anyway.

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>