onone: dri: Synchronize the cursor state

onone: dri: Synchronize the cursor state

This is preparation for moving the cursor on a dedicated input thread.
Add a mutex to protect the state of the cursor, with the goal that
simple movement is possible without involving the UI thread.

Some operations such as changing the bitmap and moving the cursor
between windows must still be done on UI thread - the window & window
manager are not synchronized and not safe for use from other threads.

BUG=446184
TEST=manual on link_freon

Review URL: https://codereview.chromium.org/846393003

Cr-Commit-Position: refs/heads/master@{#312018}