drm/amdgpu: Remove warning for virtual_display

Remove the virtual_display warning in drm_crtc_vblank_off when
dev->num_crtcs is null.
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Emily.Deng <Emily.Deng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/dce_virtual.c

@@ -174,7 +174,9 @@ static void dce_virtual_crtc_commit(struct drm_crtc *crtc)
 static void dce_virtual_crtc_disable(struct drm_crtc *crtc)
 {
 	struct amdgpu_crtc *amdgpu_crtc = to_amdgpu_crtc(crtc);
+	struct drm_device *dev = crtc->dev;
 
+	if (dev->num_crtcs)
 		drm_crtc_vblank_off(crtc);
 
 	amdgpu_crtc->enabled = false;