winemac: Use the display unit number rather than display ID for the initial display mode registry key.

On Macs with dual GPUs that automatically switch, the display ID is not stable.
It changes when the active GPU changes.  The resulted in the lookup of the
initial display mode failing and games not being able to restore it properly.

The display unit number should be more reliable, although still not perfect.
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>