drm/fb-helper: Set/clear dev->fb_helper in dummy init/fini

Set dev->fb_helper even when fbdev emulation is compiled out,
so drivers can use it to free the structure.
Clear it for consistency.

Fixes: 29ad20b2 ("drm: Add drm_device->fb_helper pointer")
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171215175119.36181-2-noralf@tronnes.org