drm/nouveau: make fbdev support really optional

Currently enabling Nouveau DRM support automatically pulls in
fbdev dependency. However this dep is unnecessary since
DRM core already handles this for us (DRM_FBDEV_EMULATION).

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Sean Paul <seanpaul@chromium.org>