drm/i915: Balance context pinning on reset cleanup

We pin when we submit to execlist queue. Balance
the pinning when the submitted queue is cleaned on reset.

Cc: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Thomas Daniel <thomas.daniel@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>