drm: zynqmp_dp: Unlock on error in zynqmp_dp_bridge_atomic_enable()

We added some locking to this function, but accidentally forgot to unlock
if zynqmp_dp_mode_configure() failed.  Use a guard lock to fix it.

Fixes: a7d5eeaa ("drm: zynqmp_dp: Add locking")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Sean Anderson <sean.anderson@linux.dev>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b4042bd9-c943-4738-a2e1-8647259137c6@stanley.mountain