fbcon: Move console_lock for register/unlink/unregister

fbcon: Move console_lock for register/unlink/unregister

Ideally console_lock becomes an implementation detail of fbcon.c and
doesn't show up anywhere in fbmem.c. We're still pretty far from that,
but at least the register/unregister code is there now.

With this the do_fb_ioctl() handler is the only code in fbmem.c still
calling console_lock().

Acked-by: Sam Ravnborg <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: Du Cheng <[email protected]>
Cc: Claudio Suarez <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Sam Ravnborg <[email protected]>
Cc: Zheyu Ma <[email protected]>
Cc: Guenter Roeck <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Zhen Lei <[email protected]>
Cc: Xiyu Yang <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]