drm/i915: drm/i915: Fix recursive calls to unmap

After the ILK vt-d workaround patches it became clear that we had
introduced a bug.  Chris Wilson tracked down the issue to recursive
calls to unmap. This happens because we try to optimize waiting on
requests by calling retire requests after the wait, which may drop the
last reference on an object and end up freeing the object (and then
unmap the object from the gtt).

After the last patch we can now choose to defer processing the retire
list.

Kudos to Chris Wilson for tracking this one down.

This patch fixes gem_unref_active_buffers from i-g-t. It was tested by
forcing do_idle_maps to true.

This also fixes tests/gem_linear_blits in intel-gpu-tools.

Reported-by:  <guang.a.yang@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=42180


Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>