drm/i915: Encourage our shrinker more when our shmemfs allocations fails

Commit 24f8e00a ("drm/i915: Prefer to report ENOMEM rather than
incur the oom for gfx allocations") made the bold decision to try and
avoid the oomkiller by reporting -ENOMEM to userspace if our allocation
failed after attempting to free enough buffer objects. In short, it
appears we were giving up too easily (even before we start wondering if
one pass of reclaim is as strong as we would like). Part of the problem
is that if we only shrink just enough pages for our expected allocation,
the likelihood of those pages becoming available to us is less than 100%
To counter-act that we ask for twice the number of pages to be made
available. Furthermore, we allow the shrinker to pull pages from the
active list in later passes.

v2: Be a little more cautious in paging out gfx buffers, and leave that
to a more balanced approach from shrink_slab(). Important when combined
with "drm/i915: Start writeback from the shrinker" as anything shrunk is
immediately swapped out and so should be more conservative.

Fixes: 24f8e00a ("drm/i915: Prefer to report ENOMEM rather than incur the oom for gfx allocations")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170609110350.1767-1-chris@chris-wilson.co.uk