drm/i915: Remove redundant check in i915_gem_obj_to_vma

drm/i915: Remove redundant check in i915_gem_obj_to_vma

No need to verify VMA belongs to GGTT since:

1. The function must return a normal VMA belonging to passed in VM.
2. There can only be one normal VMA for any VM.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1447329595-17495-1-git-send-email-tvrtko.ursulin@linux.intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>