i965: Refactor texture swizzle generation into a helper.

It's going to be reused in a second place soon.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit b5a042a657fed45264406cbd0d67fa6217a410a1)
3 files changed