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>