fbcon: Introduce wrapper for console->fb_info lookup

fbcon: Introduce wrapper for console->fb_info lookup

Half of it is protected by console_lock, but the other half is a lot
more awkward: Registration/deregistration of fbdev are serialized, but
we don't really clear out anything in con2fb_map and so there's
potential for use-after free mixups.

First step is to encapsulate the lookup.

Acked-by: Thomas Zimmermann <[email protected]>
Acked-by: Sam Ravnborg <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Du Cheng <[email protected]>
Cc: Claudio Suarez <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]