draw: fix incorrect vertex size computation in LLVM drawing code

draw: fix incorrect vertex size computation in LLVM drawing code

We were calling draw_total_vs_outputs() too early.  The call to
draw_pt_emit_prepare() could result in the vertex size changing.
So call draw_total_vs_outputs() after draw_pt_emit_prepare().

This fix would seem to be needed for the non-LLVM code as well,
but it's not obvious.  Instead, I added an assertion there to
try to catch this problem if it were to occur there.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72926
Cc: 10.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: José Fonseca <jfonseca@vmware.com>