drm/i915: add 'reclaimable' to i915 self-reclaimable page allocations

The hibernate issues that got fixed in commit 985b823b ("drm/i915:
fix hibernation since i915 self-reclaim fixes") turn out to have been
incomplete.  Vefa Bicakci tested lots of hibernate cycles, and without
the __GFP_RECLAIMABLE flag the system eventually fails to resume.

With the flag added, Vefa can apparently hibernate forever (or until he
gets bored running his automated scripts, whichever comes first).

The reclaimable flag was there originally, and was one of the flags that
were dropped (unintentionally) by commit 4bdadb97 ("drm/i915:
Selectively enable self-reclaim") that introduced all these problems,
but I didn't want to just blindly add back all the flags in commit
985b823b, and it looked like __GFP_RECLAIM wasn't necessary.  It
clearly was.

I still suspect that there is some subtle reason we're missing that
causes the problems, but __GFP_RECLAIMABLE is certainly not wrong to use
in this context, and is what the code historically used.  And we have no
idea what the causes the corruption without it.

Reported-and-tested-by: M. Vefa Bicakci <bicave@superonline.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>