drm/vmwgfx: Fix vmw_du_cursor_plane_atomic_check

Use the correct helper and also return early on helper
success rather than on helper failure.

Also explicitly return 0 in the case of no fb.

v2: Check for !fb after updating state->visible (Ville).

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com> (v1)
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-17-daniel.vetter@ffwll.ch