During the review of

commit 1f70999f
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Jan 27 22:43:07 2014 +0000

    drm/i915: Prevent recursion by retiring requests when the ring is full

Ville raised the point that our interaction with request->tail was
likely to foul up other uses elsewhere (such as hang check comparing
ACTHD against requests).

However, we also need to restore the implicit retire requests that certain
test cases depend upon (e.g. igt/gem_exec_lut_handle), this raises the
spectre that the ppgtt will randomly call i915_gpu_idle() and recurse
back into intel_ring_begin().
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=78023Reviewed-by: Brad Volkin <bradley.d.volkin@intel.com>
[danvet: Remove now unused 'tail' variable as spotted by Brad.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>