fbdev: efifb: Use driver-private screen_info for sysfs

Since commit b9cfd1d2 ("fbdev/efifb: Use screen_info pointer from device")
efifb uses a local copy of screen_info and applies its modifications
there. Adapt the sysfs attributes to also work with the custom copy
instead of the unmodified platform data.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Helge Deller <deller@gmx.de>