fbcon: set_con2fb_map fixes

set_con2fb_map() has regressed for some time.  Using fbcon=map:01, for
example, works only if there is only 1 working framebuffer. Trying to do a
set_con2fb_map() on a non-allocated vc will freeze the system.

- ensure that succeeding drivers after the first gets mapped to the console
- remove fbcon_preset_display() and modify fbcon_set_display() to include the
  former's functionality
- ensure that binding and unbinding succeeds if multiple drivers are mapped to
  the console

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>