animcur: Fix transitions between animated cursors

We weren't cancelling the old timer when changing cursors, making things
go all crashy. Logically we could always cancel the timer first, but
then we'd have to call TimerSet to re-arm ourselves, and GetTimeInMillis
is potentially expensive.

Reported-by: https://devtalk.nvidia.com/default/topic/1028172/linux/titan-v-ubuntu-16-04lts-and-387-34-driver-crashes-badly/post/5230967/#5230967


Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Tested-by: Aaron Plattner <aplattner@nvidia.com>