drm: Add drm_device->fb_helper pointer

drm_fb_helper is *the* way of doing fbdev emulation so add a pointer to
struct drm_device. This makes it possible to add callback helpers for
.last_close and .output_poll_changed further reducing fbdev emulation
footprint in drivers. The pointer is set by drm_fb_helper_init() and
cleared by drm_fb_helper_fini().

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171030153951.56269-3-noralf@tronnes.org