drm/fb-helper: Finish the generic fbdev emulation

This adds a drm_fbdev_generic_setup() function that sets up generic
fbdev emulation with client callbacks for restore, hotplug and
unregister.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703160354.59955-7-noralf@tronnes.org