drm: xlnx: db: fix a memory leak in probe

Free "dp" before returning.

Fixes: be318d01 ("drm: xlnx: dp: Reset DisplayPort IP")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/86def134-9537-4939-912e-3a424e3a75b6@moroto.mountain