engines/maxwell_*: Use nested namespace specifiers where applicable

These three source files are the only ones within the engines directory
that don't use nested namespaces. We may as well change these over to
keep things consistent.

src/video_core/engines/maxwell_dma.cpp

@@ -7,8 +7,7 @@
 #include "video_core/rasterizer_interface.h"
 #include "video_core/textures/decoders.h"
 
-namespace Tegra {
-namespace Engines {
+namespace Tegra::Engines {
 
 MaxwellDMA::MaxwellDMA(VideoCore::RasterizerInterface& rasterizer, MemoryManager& memory_manager)
     : memory_manager(memory_manager), rasterizer{rasterizer} {}
@@ -119,5 +118,4 @@ void MaxwellDMA::HandleCopy() {
     }
 }
 
-} // namespace Engines
-} // namespace Tegra
+} // namespace Tegra::Engines