drm/i915: Only reset the pinned kernel contexts on resume

On resume, we know that the only pinned contexts in danger of seeing
corruption are the kernel context, and so we do not need to walk the
list of all GEM contexts as we tracked them on each engine.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190410190120.830-1-chris@chris-wilson.co.uk