video_core/engines: Remove unnecessary inclusions where applicable

Replaces header inclusions with forward declarations where applicable
and also removes unused headers within the cpp file. This reduces a few
more dependencies on core/memory.h

src/video_core/engines/kepler_compute.h

@@ -9,7 +9,10 @@
 #include "common/common_funcs.h"
 #include "common/common_types.h"
 #include "video_core/gpu.h"
-#include "video_core/memory_manager.h"
+
+namespace Tegra {
+class MemoryManager;
+}
 
 namespace Tegra::Engines {