drm/i915/gvt: mandatory require hypervisor's host_init

Don't mark hypervisor module's host_init as optional,
but mandatory required.
Reviewed-by: Yuan, Hang <hang.yuan@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

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

@@ -52,9 +52,8 @@
 static inline int intel_gvt_hypervisor_host_init(struct device *dev,
 						 void *gvt, const void *ops)
 {
-	/* optional to provide */
 	if (!intel_gvt_host.mpt->host_init)
-		return 0;
+		return -ENODEV;
 
 	return intel_gvt_host.mpt->host_init(dev, gvt, ops);
 }