draw: simplify prim id insertion in prim assembler

draw: simplify prim id insertion in prim assembler

Because all topologies are reduced to basic primitives (i.e. no strips, fans)
and the vertices involved are all copied, there's no need for any elaborate
decisions where to insert the prim id. The logic employed was correct for
first provoking vertex, but didn't account at all for the last provoking
vertex case. And since we now will get the right constant value even if the
primitive type is later changed (for unfilled etc.) this is no longer
required to pass certain tests (which were checking for prim_id == some
const interpolated value so passing because both were wrong in the end).
This is a bit overkill (3x4 values assigned in total even though it's really
one scalar per prim...) but the code is now much easier and I don't need to
add more cases for last provoking vertex.

This fixes piglit primitive-id-no-gs-strip test.

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