st/nine: Avoid redundant SetCursorPos calls

For some applications SetCursorPosition
is called when a cursor event is received.

Our SetCursorPosition was always calling
wine SetCursorPos which would trigger
a cursor event.

The infinite loop is avoided by not calling
SetCursorPos when the position hasn't changed.
Found thanks to wine tests.

Fixes irresponsive GUI for some applications.

Fixes: https://github.com/iXit/Mesa-3D/issues/173



Signed-off-by: Axel Davy <davyaxel0@gmail.com>
CC: <mesa-stable@lists.freedesktop.org>
(cherry picked from commit dcfde02b)