drm: replace "drm_dev_unref" function with "drm_dev_put"

This patch unifies the naming of DRM functions for reference counting as
requested on Documentation/gpu/todo.rst

Signed-off-by: Fernando Ramos <greenfoo@gluegarage.com>
Acked-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181115221634.22715-4-greenfoo@gluegarage.com