drm/i915/gvt: declare gvt as i915's soft dependency

This helps initramfs builder and other tools to know the full dependencies
of i915 and have gvt module loaded with i915.

v2: add condition and change to pre-dependency (Chris)
v3: move declaration to gvt.c. (Chris)
v4: remove xengt (Zhenyu)
Signed-off-by: NHang Yuan <hang.yuan@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NZhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/gvt.c

@@ -468,3 +468,7 @@ int intel_gvt_init_device(struct drm_i915_private *dev_priv)
 	kfree(gvt);
 	return ret;
 }
+
+#if IS_ENABLED(CONFIG_DRM_I915_GVT_KVMGT)
+MODULE_SOFTDEP("pre: kvmgt");
+#endif