llvmpipe: Don't use u_ringbuffer for lp_scene_queue

llvmpipe: Don't use u_ringbuffer for lp_scene_queue

Inline the ring buffer and signal logic into lp_scene_queue instead of
using a u_ringbuffer.  The code ends up simpler since there's no need
to handle serializing data from / to packets.

This fixes a crash when compiling Mesa with LTO, that happened because
of util_ringbuffer_dequeue() was writing data after the "header
packet", as shown below

    struct scene_packet {
       struct util_packet header;
       struct lp_scene *scene;
    };

    /* Snippet of old lp_scene_deque(). */
    packet.scene = NULL;
    ret = util_ringbuffer_dequeue(queue->ring,
                                  &packet.header,
                                  sizeof packet / 4,
    return packet.scene;

but due to the way aliasing analysis work the compiler didn't
considered the "&packet->header" to alias with "packet->scene".  With
the aggressive inlining done by LTO, this would end up always
returning NULL instead of the content read by
util_ringbuffer_dequeue().

Issue found by Marco Simental and iThiago Macieira.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110884
Reviewed-by: Roland Scheidegger <sroland@vmware.com>