drm/i915/gvt: make gtt.h self-contained

Add necessary #includes and forward declarations to make the header
compile on its own.
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/ed11c982872c7c3bef4f22d13dfa029e18e980a9.1607422863.git.jani.nikula@intel.com

drivers/gpu/drm/i915/Makefile

@@ -284,7 +284,6 @@ obj-$(CONFIG_DRM_I915_GVT_KVMGT) += gvt/kvmgt.o
 # exclude some broken headers from the test coverage
 no-header-test := \
 	display/intel_vbt_defs.h \
-	gvt/gtt.h \
 	gvt/gvt.h \
 	gvt/interrupt.h \
 	gvt/mmio_context.h \

drivers/gpu/drm/i915/gvt/gtt.h

@@ -34,10 +34,19 @@
 #ifndef _GVT_GTT_H_
 #define _GVT_GTT_H_
 
-#define I915_GTT_PAGE_SHIFT         12
+#include <linux/kernel.h>
+#include <linux/kref.h>
+#include <linux/mutex.h>
+#include <linux/radix-tree.h>
+
+#include "gt/intel_gtt.h"
 
+struct intel_gvt;
+struct intel_vgpu;
 struct intel_vgpu_mm;
 
+#define I915_GTT_PAGE_SHIFT         12
+
 #define INTEL_GVT_INVALID_ADDR (~0UL)
 
 struct intel_gvt_gtt_entry {