drm/fb-cma-helper: Add fb_deferred_io support

This adds fbdev deferred io support if CONFIG_FB_DEFERRED_IO is enabled.
The driver has to provide a (struct drm_framebuffer_funcs *)->dirty()
callback to get notification of fbdev framebuffer changes.
If the dirty() hook is set, then fb_deferred_io is set up automatically
by the helper.

Two functions have been added so that the driver can provide a dirty()
function:
- drm_fbdev_cma_init_with_funcs()
  This makes it possible for the driver to provided a custom
  (struct drm_fb_helper_funcs *)->fb_probe() function.
- drm_fbdev_cma_create_with_funcs()
  This is used by the .fb_probe hook to set a driver provided
  (struct drm_framebuffer_funcs *)->dirty() function.

Cc: laurent.pinchart@ideasonboard.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461856717-6476-6-git-send-email-noralf@tronnes.org