draw: fix prim ids when there's no gs

draw: fix prim ids when there's no gs

We were resetting the prim id count for each run of the prim assembler,
hence this only worked when the draw calls were very small (the exact limit
depending on the vertex size), since larger draw calls get split up.
So, do the same as we do already if there's a gs, reset it to zero explicitly
for every new instance (this possibly could use the same variable but that
isn't doable without some heavy refactoring and I'm not sure it makes sense).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90130.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
CC: <mesa-stable@lists.freedesktop.org>