drm/i915: Simplify object is-pinned checking for shrinker

When looking for viable candidates to shrink, we only want objects that
are not pinned. However to do so we performed a double iteration over
the vma in the objects, first looking for the pin-count, then looking
for allocations. We can do both at once and be slightly more explicit in
our validity test.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>