drm/nouveau/fbcon: fix oops without fbdev emulation

This is similar to an earlier commit 52dfcc5c ("drm/nouveau: fix for
disabled fbdev emulation"), but protects all occurrences of helper.fbdev
in the source.

I see oops in nouveau_fbcon_accel_save_disable() called from
nouveau_fbcon_set_suspend_work() on Linux 3.13 when
CONFIG_DRM_FBDEV_EMULATION option is disabled.

Signed-off-by: Pavel Roskin <plroskin@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>