drm/vc4: Add a proper short-circut path for legacy cursor updates.

Previously, on every modeset we would allocate new display list
memory, recompute changed planes, write all of them to the new memory,
and pointed scanout at the new list (which will latch approximately at
the next line of scanout).  We let
drm_atomic_helper_wait_for_vblanks() decide whether we needed to wait
for a vblank after a modeset before cleaning up the old state and
letting the next modeset proceed, and on legacy cursor updates we
wouldn't wait.  If you moved the cursor fast enough, we could
potentially wrap around the display list memory area and overwrite the
existing display list while it was still being scanned out, resulting
in the HVS scanning out garbage or just halting.

Instead of making cursor updates wait for scanout to move to the new
display list area (which introduces significant cursor lag in X), we
just rewrite our current display list.

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