drm/i915/gtt: Don't try to clear failed empty pd allocation

When __gen8_ppgtt_alloc fails without allocating anything
we should not try to call __gen8_ppgtt_clear as there is
nothing to clear and underlying code will complain with:

[  157.861645] gen8_pd_range:881 GEM_BUG_ON(start >= end)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719153322.10464-1-michal.wajdeczko@intel.com