draw: fix flatshade stage for constant interpolated values

draw: fix flatshade stage for constant interpolated values

This stage only worked for traditional old-school flatshading, it did ignore
constant interpolated values and only handled colors, the code probably
predates using of constant interpolated values in gallium. So fix this - the
clip stage apparently did this a long time ago already.
Unfortunately this also means the stage needs to be invoked when flatshading
isn't enabled but some other prim changing stages are - for instance with
fill mode line each of the 3 lines in a tri should get the same attribute
value from the leading vertex in the original tri if interpolation is constant,
which did not happen before
Due to that, the stage is now run in more cases, even unnecessary ones. Could
in theory skip it completely if there aren't any constant interpolated
attributes (and rast->flatshade isn't set), but not sure it's worth bothering,
as it looks kinda complicated getting this information in advance.

No piglit change (doesn't really cover this directly).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>