drm/vmwgfx: Move the stdu vblank event to atomic function

Atomic ioctl can also send the same page flip flags as legacy ioctl.
In those cases also need to send the vblank event to userspace.

vmwgfx does not support flag DRM_MODE_PAGE_FLIP_ASYNC, so this flag is
never expected.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>