drm: bridge: Make (pre/post) enable/disable callbacks optional

Instead of forcing bridges to implement empty callbacks make them all
optional.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456480266-7904-1-git-send-email-laurent.pinchart+renesas@ideasonboard.com


Acked-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>