drm/i915: i915_gem_shrink_all() needs an awake device

drm/i915: i915_gem_shrink_all() needs an awake device

Since to unbind an object, we may need a powered up device to access the
GTT entries, we only shrink bound objects if awake. Callers to
i915_gem_shrink_all() had to take this into account and take the rpm
wakeref, but we can move this wakeref into the shrink_all itself for
convenience and making the function live up to its name.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170208104710.18089-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>