drm/bridge: lvds-encoder: Fix build error while CONFIG_DRM_KMS_HELPER=m

If DRM_LVDS_ENCODER=y but CONFIG_DRM_KMS_HELPER=m,
build fails:

drivers/gpu/drm/bridge/lvds-encoder.o: In function `lvds_encoder_probe':
lvds-encoder.c:(.text+0x155): undefined reference to `devm_drm_panel_bridge_add'

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: dbb58bfd

 ("drm/bridge: Fix lvds-encoder since the panel_bridge rework.")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190729071216.27488-1-yuehaibing@huawei.com