drm/nv50: fix ramin heap size for kernel channel too

Port change from "drm/nouveau: Keep RAMIN heap within the channel"
to kernel channel, which has its own ramin heap initialisation.
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Younes Manton <younes.m@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_instmem.c

@@ -83,7 +83,7 @@ nv50_channel_new(struct drm_device *dev, u32 size, struct nouveau_vm *vm,
 		return ret;
 	}
 
-	ret = drm_mm_init(&chan->ramin_heap, 0x6000, chan->ramin->size);
+	ret = drm_mm_init(&chan->ramin_heap, 0x6000, chan->ramin->size - 0x6000);
 	if (ret) {
 		nv50_channel_del(&chan);
 		return ret;