drm/panel: check failure cases in the probe func

The following function calls may fail and return NULL, so the null check
is added.
of_graph_get_next_endpoint
of_graph_get_remote_port_parent
of_graph_get_remote_port

Update: Thanks to Sam Ravnborg, for suggession on the use of goto to avoid
leaking endpoint.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190724195534.9303-1-navid.emamdoost@gmail.com