drm/sti: sti_vdo: clean up drm_bridge_add call

This patch removes unnecessary checking of return value.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1499071350-25168-15-git-send-email-inki.dae@samsung.com