drm/tinydrm: Use kmemdup rather than duplicating its implementation in repaper_spi_transfer()

use kmemdup rather than duplicating its implementation

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1543471233-159568-1-git-send-email-yuehaibing@huawei.com