drm/i915: Mark the current context as lost on suspend

In order to force a reload of the context image upon resume, we first
need to mark its absence on suspend. Currently we are failing to restore
the golden context state and any context w/a to the default context
after resume.

One oversight corrected, is that we had forgotten to reapply the L3
remapping when restoring the lost default context.

v2: Remove deprecated WARN.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461833819-3991-7-git-send-email-chris@chris-wilson.co.uk