fbcon: allow fbcon to use the primary display driver

Allow fbcon to select the primary display adapter using the
fb_is_primary_device() arch-specific helper.  If a a primary adapter is
detected, fbcon will unbind the old adapter from the VT layer, then rebind
using the new adapter.  This requires that bind_/unbind_con_driver() be made
public.

Because this feature may produce unexpected behavior (from the user's POV),
this must be explicitly enabled in Kconfig.

[akpm@linux-foundation.org: export unbind_con_driver]
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>