drm/i915: check context reset stats before relocations

Doing it early prevents moving and relocating objects in vain
for contexts that won't get any GPU time.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>