drm/client: Fix error code in drm_client_buffer_vmap_local()

This function accidentally returns zero/success on the failure path.
It leads to locking issues and an uninitialized *map_copy in the
caller.

Fixes: b4b0193e ("drm/fbdev-generic: Fix locking with drm_client_buffer_vmap_local()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/89d13df3-747c-4c5d-b122-d081aef5110a@stanley.mountain