drm/fb-helper: Add drm_fb_helper_fbdev_setup/teardown()

Add helpers to setup and teardown fbdev emulation.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171215175119.36181-3-noralf@tronnes.org