drm/i915: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
warning by explicitly adding a return; statement:

drivers/gpu/drm/i915/gem/i915_gem_shrinker.c:65:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]

Link: https://github.com/KSPP/linux/issues/115Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/gpu/drm/i915/gem/i915_gem_shrinker.c

@@ -62,6 +62,7 @@ static void try_to_writeback(struct drm_i915_gem_object *obj,
 	switch (obj->mm.madv) {
 	case I915_MADV_DONTNEED:
 		i915_gem_object_truncate(obj);
+		return;
 	case __I915_MADV_PURGED:
 		return;
 	}