drm/nv84: fix minor issues in PCRYPT implementation

Fix running of destroy_context() when create_context() has never been
called for the channel, and fill in engine's tlb_flush() function pointer.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nv84_crypt.c b/drivers/gpu/drm/nouveau/nv84_crypt.c
index 1cda024..780bb1d 100644
--- a/drivers/gpu/drm/nouveau/nv84_crypt.c
+++ b/drivers/gpu/drm/nouveau/nv84_crypt.c
@@ -61,7 +61,7 @@
 	struct drm_device *dev = chan->dev;
 	u32 inst;
 
-	if (!chan->ramin)
+	if (!chan->crypt_ctx)
 		return;
 
 	inst  = (chan->ramin->vinst >> 12);