compositor-drm: update internal cursor plane location when disabling cursor

compositor-drm: update internal cursor plane location when disabling cursor

When the cursor plane is disabled the kernel can lose its location.
If we don't update our internal idea of where the plane is at that time,
the next time we set a cursor it can show up at 0,0.

This can show up when an application is put in the cursor plane, removed
from the plane, then put back at the same location.  It might show up at
0,0 when it's reinstated.

We now use INT32_MIN as a location for disabled cursors so enabling the
plane will always cause an update.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>