fbdev: find mode with the highest/safest refresh rate in fb_find_mode()

Currently, if the refresh rate is not specified, fb_find_mode() returns the
first known video mode with the requested resolution, which provides no
guarantees wrt the refresh rate.  Change this so that the mode with the
highest refresh rate is returned when the driver provides a custom video mode
database and the monitor limits, and a mode with the safe 60 Hz refresh rate
otherwise.

Signed-off-by: Michal Januszewski <spock@gentoo.org>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>