llvmpipe: Use the default behavior of ALLOW_MAPPED_BUFFERS.

Since this is a software rasterizer, we really don't care whether the
buffers are "mapped" since it's just malloc.  This will drop a bit of
pointless CPU overhead to throw errors.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3298>