mesa: Correct the is_vertex_position decision for dlists.

mesa: Correct the is_vertex_position decision for dlists.

We have to use _mesa_inside_dlist_begin_end instead of
_mesa_inside_begin_end to see if we are inside a glBegin/glEnd block in
case of display lists.
So split the is_vertex_position function used in vertex attribute processing
into a imm and dlist variant and use the appropriate _mesa_inside_begin_end
variant.

Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>