drm/bridge/synopsys: dw-hdmi: Fix unwedge crash when no pinctrl entries

In commit 50f9495e ("drm/bridge/synopsys: dw-hdmi: Add "unwedge"
for ddc bus") I stupidly used IS_ERR() to check for whether we have an
"unwedge" pinctrl state even though on most flows through the driver
the unwedge state will just be NULL.

Fix it so that we consistently use NULL for no unwedge state.

Fixes: 50f9495e ("drm/bridge/synopsys: dw-hdmi: Add "unwedge" for ddc bus")
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Jernej Skrabec <jernej.skrabec@siol.net>
Cc: Sam Ravnborg <sam@ravnborg.org>
Reported-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190610175234.196844-1-dianders@chromium.org