drm/vmwgfx: Fix a boot time warning

The 4.15 vmwgfx driver shows a warning during boot.
It is caused by a mismatch between the result of vmw_enable_vblank()
and what the drm_atomic_helper expects.

Signed-off by: Woody Suwalski <terraluna977@gmail.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_kms.c

@@ -1863,7 +1863,7 @@ u32 vmw_get_vblank_counter(struct drm_device *dev, unsigned int pipe)
  */
 int vmw_enable_vblank(struct drm_device *dev, unsigned int pipe)
 {
-	return -ENOSYS;
+	return -EINVAL;
 }
 
 /**