fbdev: stifb: Provide valid pixelclock and add fb_check_var() checks

commit 203873a535d627c668f293be0cb73e26c30f9cc7 upstream.

Find a valid modeline depending on the machine graphic card
configuration and add the fb_check_var() function to validate
Xorg provided graphics settings.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>