mesa: correctly allocate space for converted primtives

mesa: correctly allocate space for converted primtives

Previosuly it was assumed that primitives where always converted to
triangles if the driver did not support all primitives, however that's
not true for a driver that supports quads but not quad strips.

Fixes piglit spec@!opengl 1.1@dlist-fdo3129-01 on Panfrost

Fixes: dcbf2423d28 ("vbo/dlist: add vertices to incomplete primitives")
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21987>