drm/rockchip: Avoid uninitialized use of endpoint id in LVDS

In the Rockchip DRM LVDS component driver, the endpoint id provided to
drm_of_find_panel_or_bridge is grabbed from the endpoint's reg property.

However, the property may be missing in the case of a single endpoint.
Initialize the endpoint_id variable to 0 to avoid using an
uninitialized variable in that case.

Fixes: 34cc0aa2 ("drm/rockchip: Add support for Rockchip Soc LVDS")
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201110200430.1713467-1-paul.kocialkowski@bootlin.com