i965: Fix flat integral varyings.

i965: Fix flat integral varyings.

Previously, the vertex and fragment shader back-ends assumed that all
varyings were floats.  In GLSL 1.30 this is no longer true--they can
also be of integral types provided that they have an interpolation
qualifier of "flat".

This required two changes in each back-end: assigning the correct type
to the register that holds the varying value during shader execution,
and assigning the correct type to the register that ties the varying
value to the rest of the graphics pipeline (the message register in
the case of VS, and the payload register in the case of FS).

Fixes piglit tests fs-int-interpolation and fs-uint-interpolation.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>