bridge: Fix panel-bridge error return on !panel.

ERR_PTR() needs a negative errno argument.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170615175423.17954-1-eric@anholt.net