Silence several build warnings (#1428)

* Silence several build warnings

* Remove fixed buffers from NVDEC struct

* Remove unused field and usings

* Fix wrong name

* Silence more warning on H264 PictureInfo

Ryujinx.Graphics.Host1x/Host1xClassRegisters.cs

@@ -4,6 +4,7 @@ namespace Ryujinx.Graphics.Host1x
 {
     struct Host1xClassRegisters
     {
+#pragma warning disable CS0649
         public uint IncrSyncpt;
         public uint IncrSyncptCntrl;
         public uint IncrSyncptError;
@@ -37,5 +38,6 @@ namespace Ryujinx.Graphics.Host1x
         public uint Xrefctrl;
         public uint ChannelXrefHi;
         public uint ChannelXrefLo;
+#pragma warning restore CS0649
     }
 }

Ryujinx.Graphics.Host1x/ThiRegisters.cs

@@ -4,6 +4,7 @@ namespace Ryujinx.Graphics.Host1x
 {
     struct ThiRegisters
     {
+#pragma warning disable CS0649
         public uint IncrSyncpt;
         public uint Reserved4;
         public uint IncrSyncptErr;
@@ -18,5 +19,6 @@ namespace Ryujinx.Graphics.Host1x
         public Array12<uint> Reserved48;
         public uint IntStatus;
         public uint IntMask;
+#pragma warning restore CS0649
     }
 }