i965/msaa: Add backend support for centroid interpolation.

i965/msaa: Add backend support for centroid interpolation.

This patch causes the fragment shader to be configured correctly (and
the correct code to be generated) for centroid interpolation.  This
required two changes: brw_compute_barycentric_interp_modes() needs to
determine when centroid barycentric coordinates need to be included in
the pixel shader thread payload, and
fs_visitor::emit_general_interpolation() needs to interpolate using
the correct set of barycentric coordinates.

Fixes piglit tests "EXT_framebuffer_multisample/interpolation {2,4}
centroid-edges" on i965.

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