drm/i915: Allow kswapd to pause the device whilst reaping

drm/i915: Allow kswapd to pause the device whilst reaping

In commit 5763ff04dc4e ("drm/i915: Avoid GPU stalls from kswapd") we
stopped direct reclaim and kswapd from triggering GPU/client stalls
whilst running (by restricting the objects they could reap to be idle).

However with abusive GPU usage, it becomes quite easy to starve kswapd
of memory and prevent it from making forward progress towards obtaining
enough free memory (thus driving the system closer to swap exhaustion).
Relax the previous restriction to allow kswapd (but not direct reclaim)
to stall the device whilst reaping purgeable pages.

v2: Also acquire the rpm wakelock to allow kswapd to unbind buffers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170601133331.5973-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>