drm: remove deprecated "[__]drm_gem_object_[un]reference[_locked]" functions

There are no more places where these (deprecated) functions are being
used from, thus they 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/20181115221634.22715-3-greenfoo@gluegarage.com