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

Add necessary #includes and forward declarations to make the header
compile on its own.

While at it, also remove a useless forward declaration.
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/46c2acf80f1d214182a96a97fa6322ab89a3de05.1607422863.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/fb_decoder.h \
 	gvt/gtt.h \
 	gvt/gvt.h \
 	gvt/interrupt.h \

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

@@ -38,6 +38,10 @@
 
 #include <linux/types.h>
 
+#include "display/intel_display.h"
+
+struct intel_vgpu;
+
 #define _PLANE_CTL_FORMAT_SHIFT		24
 #define _PLANE_CTL_TILED_SHIFT		10
 #define _PIPE_V_SRCSZ_SHIFT		0
@@ -98,8 +102,6 @@ enum DDI_PORT {
 	DDI_PORT_E	= 4
 };
 
-struct intel_gvt;
-
 /* color space conversion and gamma correction are not included */
 struct intel_vgpu_primary_plane_format {
 	u8	enabled;	/* plane is enabled */