mesa: change _mesa_inside_dlist_begin_end() to handle PRIM_UNKNOWN

mesa: change _mesa_inside_dlist_begin_end() to handle PRIM_UNKNOWN

If the currently compiled primitive state is PRIM_UNKNOWN we should
not return true from _mesa_inside_dlist_begin_end().  This lets us
simplify the calls to that function.

Note, the call to _mesa_inside_dlist_begin_end() in vbo_save_EndList()
should have probably been checking for PRIM_UNKNOWN too, but it wasn't.
So there's no code change change.

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