fbdev: viafb: check ioremap return value in viafb_lcd_get_mobile_state

The function viafb_lcd_get_mobile_state() calls ioremap() without
checking the return value. If ioremap() fails (returns NULL), the
subsequent readw() will cause a NULL pointer dereference.

Signed-off-by: Wang Jun <1742789905@qq.com>
Signed-off-by: Helge Deller <deller@gmx.de>