fbdev/hyperv_fb: Do not clear global screen_info

Do not clear the global instance of screen_info. If necessary, clearing
fields in screen_info should be done by architecture or firmware code
that maintains the firmware framebuffer.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Helge Deller <deller@gmx.de>