ui/cocoa.m: blinky mouse cursor fix

The mouse cursor can become blinky when being moved a lot. This patch fixes that
problem by issuing the redraw sooner.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: AAA87DD7-EC20-4F4B-B71E-C38461D9FCBA@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>