i965/fs: Don't try to use bogus interpolation modes pre-Gen6.

i965/fs: Don't try to use bogus interpolation modes pre-Gen6.

Interpolation modes other than perspective-barycentric-pixel-center (and
their associated coefficients in the WM payload) only exist in Gen6 and
later.

Unfortunately, if a varying was declared as `centroid`, we would blindly
read the nonexistant values, and so produce all manner of bad behavior
-- texture swimming, snow, etc.

Fixes rendering in Counter-Strike Source and Team Fortress 2 on
Ironlake.

NOTE: This is a candidate for the 9.1 branch.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Jordan Justen <jordan.l.justen@intel.com>