drm/i915: Only unbind vgacon, not other console drivers

The console subsystem only provides a function to switch to a given
console, but we want to actually only switach away from vgacon.
Unconditionally switching to the dummy console resulted in switching
away from fbcon in multi-gpu setups when other gpu drivers are loaded
before i915.

Then either the reinitialization of fbcon when i915 registers its
fbdev emulation or the teardown of the fbcon driver killed the
machine. So only switch to the dummy console when it's required.

Kudos to Chris for the original idea, I've only refined it a bit to
still unregister vgacon even when it's currently unused.

This regression has been introduced in

commit a4de0526
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jun 5 16:20:46 2014 +0200

    drm/i915: Kick out vga console

Reported-and-tested-by: Ed Tomlinson <edt@aei.ca>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>