i965: Set vertical alignment unit to 4 on Broadwell.

i965: Set vertical alignment unit to 4 on Broadwell.

Broadwell doesn't support a surface vertical alignment of 2.  It only
supports VALIGN_4, VALIGN_8, or VALIGN_16.  I chose 4 since it's the
least wasteful.

v2: Replace my comment with a better one from Eric.  Move Broadwell
    checks earlier so it's more obvious that "return 2" won't be hit.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>