drm: Remove redundant of_device_is_available check

This check is already performed by of_graph_get_remote_node. No
need to repeat it immediately after the call.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920172914.4015180-1-megous@megous.com