NvHostChannelIoctl: Implement setter for SetSubmitTimeout, SetPriority and SetTimeslice (#747)

- Implement accurate setter for SetPriority.
- Implement accurate setter for SetTimeslice (close #666).
- Implement basic setter for SetSubmitTimeout (close #678).

(plus some comments and a missing `PrintStub` call)

Ryujinx.HLE/HOS/Services/Nv/NvHostChannel/NvChannelPriority.cs

@@ -0,0 +1,9 @@
+namespace Ryujinx.HLE.HOS.Services.Nv.NvHostChannel
+{
+    enum NvChannelPriority
+    {
+        Low    = 50,
+        Medium = 100,
+        High   = 150
+    }
+}