drm/i915: optionally disable shrinker lock stealing

commit 5774506f
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Nov 21 13:04:04 2012 +0000

    drm/i915: Borrow our struct_mutex for the direct reclaim

added a nice trick to steal the struct_mutex lock in the shrinker if
it's the current task holding it. But this also caused the requirement
that every place which allocates memory needs to be careful about the
gem state of objects, since the shrinker could have pulled the rug out
from under it. We've usually solved this by carefully preallocating
things or ensure that buffers are pinned already.

But the shrinker also reaps mmap offset, so allocating those needs to
be careful, too. Now that code has been factored out into some common
helpers, so either we have fragile code depending upon the common
helper not doing something we don't want it to do. Or we need to
reimplement the mmap offset creation and so also leak implementation
details into our code.

Since this all results in leaky abstraction, cop out by disabling the
lock borrowing trick while calling down into the helpers. That way our
craziness is nicely confined to files in drm/i915.

v2: Split out the change to create_mmap_offset as request by Chris Wilson.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>