drm/bridge: change return type of drm_bridge_add function

This patch changes return type of drm_bridge_add function.

This function never return negative value but returns only 0.
So it changes the return type of this function to void one.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1499071350-25168-2-git-send-email-inki.dae@samsung.com