drm/omap: dmm_tiler: fix return error code in omap_dmm_probe()

Return -ENOMEM when allocating refill memory failed.

Fixes: 71e8831f ("drm/omap: DMM/TILER support for OMAP4+ platform")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117061045.3452287-1-yangyingliang@huawei.com