fbdev/sis: Remove dependency on screen_info

When built-in, the sis driver tries to detect the current display mode
from the global screen_info state. That state is only for architecture
and firmware code. Drivers should not use it directly as it's not
guaranteed to contain valid information.

Remove the mode-detection code from sis. Drivers that want to detect a
pre-set mode on probe should read the hardware registers directly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>