vgacon: dummy implementation for vgacon_text_force

This allows us to ditch a ton of ugly #ifdefs from a bunch of drm modeset
drivers.

v2: Make the dummy function actually return a sane value, spotted by
Ville.

v3: Because the patch is still in limbo there's no more drivers to
convert, noticed by Emil.

v4: Rebase once more, because hooray. I'll just go ahead an apply this
one later on to drm-misc.

Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>