drm: bridge: it66121: Check drm_bridge_attach retval

The return value of drm_bridge_attach() is ignored during
the it66121_bridge_attach() call, which is incorrect.

Fixes: 988156dc ("drm: bridge: add it66121 driver")
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210805185039.402178-1-robert.foss@linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210805185039.402178-1-robert.foss@linaro.org