i965: add helper for creating packing writemask

i965: add helper for creating packing writemask

For example where n=3 first_component=1 this will give us
0xE (WRITEMASK_YZW).

V2:
Add assert to check first component is <= 4 (Suggested by Ken)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>