dlist: upload vertices in compile_vertex_list

dlist: upload vertices in compile_vertex_list

Previously vertices were uploaded on-the-fly: each time
the position attribute was set, the newly added vertex
was copied to the mapped bo.

Replace this with a plain RAM buffer, and do the upload
at the end of compile_vertex_list.

This allows to remove the we-need-to-unmap-the-buffer-
before-drawing special case, but more importantly it
will allow to implement vertices deduplication in the
next commit.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11927>