i965: Move Gen4-5 interpolation stuff to brw_wm_prog_data.

i965: Move Gen4-5 interpolation stuff to brw_wm_prog_data.

This fixes glxgears rendering, which had surprisingly been broken since
late October!  Specifically, commit 91d61fbf7cb61a44adcaae51ee08ad0dd6b.

glxgears uses glShadeModel(GL_FLAT) when drawing the main portion of the
gears, then uses glShadeModel(GL_SMOOTH) for drawing the Gouraud-shaded
inner portion of the gears.  This results in the same fragment program
having two different state-dependent interpolation maps: one where
gl_Color is flat, and another where it's smooth.

The problem is that there's only one gen4_fragment_program, so it can't
store both.  Each FS compile would trash the last one.  But, the FS
compiles are cached, so the first one would store FLAT, and the second
would see a matching program in the cache and never bother to compile
one with SMOOTH.  (Clearing the program cache on every draw made it
render correctly.)

Instead, move it to brw_wm_prog_data, where we can keep a copy for
every specialization of the program.  The only downside is bloating
the structure a bit, but we can tighten that up a bit if we need to.
This also lets us kill gen4_fragment_program entirely!

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>