draw: fix another decompose bug affecting constant interpolated attributes

draw: fix another decompose bug affecting constant interpolated attributes

Previously the first provoking vertex convention would only be used if
flatshading were enabled. No matter how I look at it that cannot be possibly
correct. Maybe the code getting used was somewhat simpler that way at a time
where there weren't constant interpolated attributes, only flatshading...
(Note that all other places including the decomposition macros already do
the same.)

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