drm/i915: make intel_gvt.h self-contained

Add the minimal includes/declarations to make the header self-contained,
and ensure it stays that way.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626144020.2155-12-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_gvt.h

@@ -24,7 +24,7 @@
 #ifndef _INTEL_GVT_H_
 #define _INTEL_GVT_H_
 
-struct intel_gvt;
+struct drm_i915_private;
 
 #ifdef CONFIG_DRM_I915_GVT
 int intel_gvt_init(struct drm_i915_private *dev_priv);