Fix use of HASH_FIND() in RemoveScreenPointerMapping()

Replace HASH_FIND() with the convenience macro HASH_FIND_PTR() in
RemoveScreenPointerMapping() and ScreenFromPointer(), to be consistent
with AddScreenPointerMapping().  This has the side-effect of fixing
invalid usage of the original HASH_FIND() macro in
RemoveScreenPointerMapping().

Signed-Off-By: Brian Nguyen <brnguyen@nvidia.com>
Fixes #3