drm/vc4: Don't wait for vblank on fkms cursor updates.

We don't use the same async update path between fkms and normal kms,
and the normal kms workaround ended up making us wait.  This became a
larger problem in rpi-4.14.y, as the USB HID update rate throttling
got (accidentally?) dropped.

Signed-off-by: Eric Anholt <eric@anholt.net>