drm/i915: Remove wrong warning from i915_gem_context_clean

commit e9f24d5f
Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Date:   Mon Oct 5 13:26:36 2015 +0100

    drm/i915: Clean up associated VMAs on context destruction

Introduced a wrong assumption that all contexts have a ppgtt
instance. This is not true when full PPGTT is not active so
remove the WARN_ON_ONCE from the context cleanup code.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>