drm/bridge/lontium-lt9611uxc: fix get_edid return code

Return NULL pointer from get_edid() callback rather than ERR_PTR()
pointer, as DRM code does NULL checks rather than IS_ERR(). Also while
we are at it, return NULL if getting EDID timed out.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Fixes: 0cbbd5b1

 ("drm: bridge: add support for lontium LT9611UXC bridge")
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210121233303.1221784-3-dmitry.baryshkov@linaro.org