drm/i915: BUG_ON when ggtt_view is NULL

Lets BUG_ON and don't bother with a WARN and returning an error, so we can
remove the need to pollute the code with error handling, after all it is
a programmer error to provide NULL view. Also while we're here remove
redundant NULL ggtt_view check.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458834860-7898-1-git-send-email-matthew.auld@intel.com
diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index 7cfafdc..073c6bb 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -3341,15 +3341,7 @@
 				       const struct i915_ggtt_view *view)
 {
 	struct i915_address_space *ggtt = i915_obj_to_ggtt(obj);
-	struct i915_vma *vma;
-
-	if (WARN_ON(!view))
-		return ERR_PTR(-EINVAL);
-
-	vma = i915_gem_obj_to_ggtt_view(obj, view);
-
-	if (IS_ERR(vma))
-		return vma;
+	struct i915_vma *vma = i915_gem_obj_to_ggtt_view(obj, view);
 
 	if (!vma)
 		vma = __i915_gem_vma_create(obj, ggtt, view);