tests/gem_pipe_control_store_loop: Add subtest for reused buffers

tests/gem_pipe_control_store_loop: Add subtest for reused buffers

This exercises the slightly faulty kernel w/a that Eric fixed in

commit e844b990b1df9242bb91b7d490552f3198946838
Author: Eric Anholt <eric@anholt.net>
Date:   Tue Jul 31 15:35:01 2012 -0700

    drm/i915: Don't forget to apply SNB PIPE_CONTROL GTT workaround.

    If a buffer that was the target of a PIPE_CONTROL from userland was a
    reused one that hadn't been evicted which had not previously had this
    workaround applied, then the early return for a correct
    presumed_offset in this function meant we would not bind it into the
    GTT and the write would land somewhere else.

    Fixes reproducible failures with GL_EXT_timer_query usage in apitrace,
    and I also expect it to fix the intermittent OQ issues on snb that
    danvet's been working on.

    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=48019
    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=52932
    Signed-off-by: Eric Anholt <eric@anholt.net>
    Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
    Reviewed-by: Carl Worth <cworth@cworth.org>
    Tested-by: Carl Worth <cworth@cworth.org>
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>