i965/gen6+: Parameterize barycentric interpolation modes.

i965/gen6+: Parameterize barycentric interpolation modes.

This patch modifies the fragment shader back-end so that instead of
using a single delta_x/delta_y register pair to store barycentric
coordinates, it uses an array of such register pairs, one for each
possible intepolation mode.

When setting up the WM, we intstruct it to only provide the
barycentric coordinates that are actually needed by the fragment
shader--that is computed by brw_compute_barycentric_interp_modes().
Currently this function returns just
BRW_WM_PERSPECTIVE_PIXEL_BARYCENTRIC, because this is the only
interpolation mode we support.  However, that will change in a later
patch.

Reviewed-by: Eric Anholt <eric@anholt.net>