fbdev: Wrap fbcon updates from vga-switcheroo in helper

fbdev: Wrap fbcon updates from vga-switcheroo in helper

Handle console remapping in fbcon in fb_switch_output(). Vga-switcheroo
invokes this functionality before switching physical outputs to a new
graphics device. Open-coding fbcon state in vga-switcheroo exposed fbdev
implementation details.

Vga-switcheroo is used for switching physical outputs among graphics
hardware. This functionality is only supported by DRM drivers. A later
update will further move fb_switch_output() into DRM's fbdev emulation;
thus fully decoupling vga-switcheroo from fbdev.

v3:
- remove Kconfig dependency related to fbcon (Geert)
v2:
- use '#if defined' (Helge)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>