Use window's cursor when pointer enters instead of overriding it

Otherwise, it will override cursors for whole window with default
cursor. Also don't care about pointer_leave. Compositor or other
surface which pointer will enter in will handle the cursor when
pointer leaves.

Change-Id: I05e84d427d46d71d5ee38ebbd0647c40a99ef351
Reviewed-by: Andy Nichols <andy.nichols@digia.com>