draw: use the prim type from prim_info not emit in passthrough emit

draw: use the prim type from prim_info not emit in passthrough emit

The prim assembler may change the prim type when injecting prim ids now,
which isn't reflected by what's stored in emit.
This looks brittle and potentially dangerous (it is not obvious if such prim
type changes are really supported by pt emit, the prim type is actually also
set in prepare which would then be different).

This fixes piglit primitive-id-no-gs-first-vertex.shader_test.

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