drm/vmwgfx: Use a mutex to protect gui positioning in vmw_display_unit

To avoid race condition between update_layout ioctl and modeset ioctl
for access to gui_x/y positioning added a new mutex
requested_layout_mutex.

Also used drm_for_each_connector_iter to iterate over connector list.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>