drm/i915: Execlists cannot pin a context without the object

Given that the intel_lr_context_pin cannot succeed without the object,
we cannot reach intel_lr_context_unpin() without first allocating that
object - so we can remove the redundant test.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456485751-15213-1-git-send-email-tvrtko.ursulin@linux.intel.com