drm/vmwgfx: Cleanup multimon initialization code

The results of the legacy display unit initialization were being silently
ignored. Unifying the selection of number of display units based
on whether the underlying device supports multimon makes it easier
to add error checking to all paths.

This makes the driver report the errors in ldu initialization paths
and try to recover from them.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Maaz Mombasawala <mombasawalam@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220302152426.885214-3-zack@kde.org