firmware/sysfb: Clear screen_info state after consuming it

After consuming the global screen_info_state in sysfb_init(), the
created platform device maintains the firmware framebuffer. Clear
screen_info to avoid conflicting access. Subsequent kexec reboots
now ignore 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>