drm-backend: Fix cursor updates with overlapping heads

We can't use the surface damage to determine when to upload new cursor
images because when heads overlap the first repainted head will accumulate
that damage as plane damage.

We can't easily use plane damage either because the plane isn't really
assigned until after an atomic test, which requires the cursor fb to be
current.

Untangle this mess a little by always testing with the first cursor fb,
which is identical to the second in all ways, then replace with the correct
fb in repaint.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>