i965: Clarify nomenclature: vert_result -> varying

This patch removes the terminology "vert_result" from the i965 driver,
replacing it with "varying".  The old terminology, "vert_result", was
confusing because (a) it referred to the enum gl_vert_result, which no
longer exists (it was replaced with gl_varying_slot), and (b) it
implied a vertex output, but with the advent of geometry shaders, it
could be either a vertex or a geometry output, depending what shaders
are in use.  The generic term "varying" is less confusing.

No functional change.