mesa: Fold error generation into _mesa_valid_prim_mode().

mesa: Fold error generation into _mesa_valid_prim_mode().

We want to start emitting an INVALID_OPERATION from here for transform
feedback.  Note that this forced dlist.c to almost not use this
function, since it wants different behavior during dlist compile.
Just pull the non-TF, non-GS test out for compile, because:

1) TF doesn't matter in that case because there's no drawing.
2) I don't think we're going to see GSes and display lists in the same
   context, if we don't do GL_ARB_compatibility.

Reviewed-by: Brian Paul <brianp@vmware.com>