drm: remove deprecated "drm_dev_unref" function

There are no more places where this (deprecated) function is being used
from, thus it can now be removed.

Signed-off-by: Fernando Ramos <greenfoo@gluegarage.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181115221634.22715-5-greenfoo@gluegarage.com