fbcon: check console-fb mapping in fbcon_get_requirement

- Check the console-to-fb mapping in fbcon_get_requirement(), otherwise the
  value returned may not be valid for the driver.

- Minor cleanup

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>